Closes #1070.

This PR:

- updates `jekyll-anchor-headings` to `1.0.12`; this was a simple copy-paste
- updates `lunr.js` to `2.3.9`; this was a bit more involved:
    - I didn't see a minified build in the repo, so I ran it through [DigitalOcean's minifier](https://www.digitalocean.com/community/tools/minify)
    - copyright notices weren't properly included in the previous minified build, so I:
        - include an actual copy of the original MIT License for `lunr.js`
        - includes proper attribution for other functions, which include derivative works

There's a tiny bundle size increase in `lunr.js` due to the comments, but I think that's reasonable given that it's related to licensing; still trivial in the grand scheme of things.

As an aside: it would be neat if we could include minification as part of the build pipeline instead!  

* Issue #1023 - note that GA4 properties are supported

* Issue #1023 - parameterize Google Analytics property script

* Issue #1023 - support a list of multiple Google Analytics tracking IDs in config

* Issue #1023 - update Google Analytics configuration doc

* Fix configuration of multiple Google Analytics properties and simplify type checking

* simplify unnecessary code repetition

* tweak Google Analytics config documentation wording

* Add 'reversed' as the preferred keyword, with 'desc' as a deprecated alternate

* Doc updates

* Add the test for 'reversed' to the toc_list

Add also a comment about this.

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

* Removes `favicon.html`, shifts content to `head_custom.html`
* explicit callout for custom favicon in customization docs
* Cleaner and more consistent documentation (review from @pdmosses)

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

In case 'just-the-docs' is used as a theme, and a user does not
eplicitly include 'jekyll-seo-tag' inside the plugins list in
_config.yml, two 'title' elements were rendered. Since
jekyll-seo-tag is always available, because it's set in the plugins
list of the theme itself, the 'seo' tag will aways work, making
the seperate title and description elements obsolete.

Fixes #1008

Fix #1020

- Move the display of nav external links from `_includes/nav.html` to `_layouts/default.html`.
- Replace ` unless include.key` by `if site.nav_external_links`.
- Wrap the body of `if site.nav_external_links` in `<ul class="nav-list">…</ul>`.

To test this PR:

1.  Add to `_config.yml`:
    ```yaml
    defaults:
      -
        scope: {path: ""}
        values: {nav_exclude: true}
    ```

2.  Check that the only link to appear in the nav panel is external.

Co-authored-by: Matt Wang <matt@matthewwang.me>

* Fix top-level active link

Fixes #1014

1.  Reverse the order of `page.parent == node.title or page.grand_parent == node.title`. This makes no difference.
2.  Replace `page.parent == node.title` by `page.parent == node.title  and page.grand_parent == nil`. The condition is evaluated first because it is rightmost.
    We have `node in first_level_pages`.
    The old condition holds not only when `page` is a child of `node`,
    but also when `page` is a grandchild of a *different* top-level page and its parent happens to have the same title as `node`.
    The new condition never holds for a grandchild.

This change has been tested locally: in v0.4.0.rc3, when the 3rd-level page `G` was selected, the link to the top-level page `F` was active, and the link to its child `G` was shown; after making the change, it is no longer active, so the link to its child `G` is not shown.

* Update nav.html

Add a comment to clarify just when top-level nodes are active.

This way, custom styles are able to override the callout styles, which were previously included last.

The regression tests in https://just-the-docs.github.io/just-the-docs-tests/navigation/grandparent/index/ include a section "A grandchild with the same parent title as a child or top-level page". Its last item fails, as can currently be seen at https://just-the-docs.github.io/just-the-docs-tests/navigation/grandparent/f/ : the first occurrence of `G` links to the grandchild of `E`., and shouldn't be included.

The regression can be fixed by changing `where: "grand_parent", page.parent` to `where_exp: "item", "item.grand_parent == page.parent"`. This should be apparent  when running the regressions tests with this PR branch as remote theme.

The two filters give the same results in Jekyll 4, but not in Jekyll 3. It seems that this was overlooked because the sanity-testing of v0.4.0.rc3 was inadvertently using Jekyll 4.2.2.

Fix external links and collections

The navigation should only display the external links once, after the links to pages that are not in collections.

The test for PR #876 at https://just-the-docs.github.io/just-the-docs-tests/navigation/external-links fails with v0.4.0.rc3,
and succeeds when the updated `nav.html` is added locally.

The docs need updating to clarify how the interaction between the collections feature and the external links feature is resolved.

* Optimize simple navigation cases

Fix inefficiency reported in feedback on v0.4.0.rc2 (see discussion #958).

This PR:

* essentially reverts `_includes/nav.html` to v0.4.0.rc1
* preserves the ARIA labels added by #950
* adds a test to optimize builds of sites that rely on `title` fields to order pages.

Building the `endoflife.date` site (130 pages) now takes only about 7 seconds.

Building the `machinetranslate.org` site ( 350 pages) takes about 7 minutes. (Without the added test, it takes just over 5 minutes: the condition of the test is merely to compare the size of two arrays, but that is apparently enough to prevent Jekyll from applying some optimization).

A warning is added to the docs about the need for numbers to be in quotes when used as title values.

* Update navigation-structure.md

A clarification is added to the docs about the need for numbers to be in quotes when used as title values.

* Simplify the control and data flow

- Defer concatenation of `string_order_pages` with `title_order_pages` until needed.
- Replace tests on size with tests for `empty`.
- Rename variables accordingly.

* Fix child nav order

This PR started from the navigation in RC1. Some cosmetic improvements had been made in RC2. This commit adds some of those changes to this PR.

It also fixes a bug (revealed by a new regression test) due to a reference to `node.child_nav_order` instead of `child.child_nav_order`, which prevented reversal of the order in children of children. Presumably a top-level reversal should apply only to direct children, and not to grandchildren. The latter interpretation would be very confusing in a deep multi-level hierarchy.

* Allow pages with numeric titles

An omitted `nav_order` value should default to the `title` value, regardless of its type. Jekyll 3 gives build errors when numbers and strings are sorted together. This commit drops the assumption that `title` values are always strings – a 404 page naturally has a numeric title. It updates the docs page accordingly.

The extra code does not affect the build time for the `endoflife.date` site (7 seconds). For the `machinetranslate` site, changing the title of the 404 page to a number increases the build time from 7 minutes to 9 minutes – the `nav_order` numbers on that site are program-generated in the range 1..1000, which might be atypical.

This commit has not yet been checked using the regression tests.

The gemspec used for testing specifies `spec.add_runtime_dependency "jekyll", "~> 3.8.5"`, and `Gemfile.lock` shows `jekyll (3.8.7)`.

* Update nav.html

Add comment about an optimization that will be possible in Jekyll 4.

* Update nav.html

- Update the comment about optimization possibility.
- TEMPORARILY add Jekyll 3 code for conditionally optimizing.

* Update nav.html

Minor improvements and cosmetic changes.

* Major revision

This update is based on extensive experimentation and profiling with alternative versions of the Liquid code used to build the main navigation panel.

Due to the fragility of Jekyll's optimizations, combining alternative approaches with conditionals turned out to be too expensive: merely adding a condition to check whether some array of pages is empty can add about 20% to the build time!

The current code avoids sorting pages on `nav_order` and `title` fields together. The standard way of doing that in Jekyll is to use the `group_by` filter; but extracting the sorted pages from the groups turned out to be too inefficient (as seen in RC1), as was generating links directly from the groups (in RC2).

Making all pages with `nav_order` values come before all those ordered by their `title` values is not ideal (it doesn't support tweaking the relative order of two pages in a list of pages ordered by their titles) but it appears to be necessary for efficient builds on large sites.

This version has not yet been fully tested for regression, but otherwise seems to give the expected navigation on the endoflife.date and machinetranslate websites. (I'm unable to install the Python-based how2data repository on my laptop, due to package version issues on Apple silicon).

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

* Fix default highlighting in custom schemes

Fix #982

The variable settings for highlighting in the default `light` scheme are currently (v0.4.0.rc2`) in `_sass/color_schemes/light.scss`.
This PR ensures that custom schemes are based on the `light` scheme.

It also adds a note explaining the default to the customization docs,
and gives an example of how to define a custom scheme based on the `dark` scheme

* Prettier

* Deleted test file

Co-authored-by: Matt Wang <matt@matthewwang.me>

Closes #961. Replays a previous attempt.

Closes #963.

- redundant `role="navigation"` on nav elements
- redundant `type="text/javascript"` on script tags
- misplaced mermaid script tag

Note that [running the validator on the deploy preview](https://validator.w3.org/nu/?doc=https%3A%2F%2Fdeploy-preview-964--just-the-docs.netlify.app%2F) has no errors now!

An occurrence of `grandchild` was miss-spelled `grand_child` (!).
That caused omission of the `active` class in the link in the nav panel for all grandchildren.

This bug was discovered using `diff` to compare the generated sites built using v0.4.0.rc1 and commit  4396b6b1c836f354bb7aa7edc1a06a49f137d615 on the fix-nav-performance PR branch.

Fix #863.

The current Liquid code to generate the navigation panel involves the inefficient extraction of a list of pages from a list of page groups (identified by @captn3m0 in his original [explanation of the performance issue](https://github.com/just-the-docs/just-the-docs/issues/863)). 

The optimisation implemented by this PR generates navigation links directly from the list of page groups, thereby avoiding the extraction of a list of pages from it. The Liquid code is now a bit tedious, but I don't see a simpler solution. 

The need for grouping pages arises because Jekyll doesn't provide a filter to sort a list of pages on the value of an arbitrary expression.

Using Jekyll v4.2.2 (macOS 12.5, M2 MacBook Air, 16 GB memory), building https://github.com/endoflife-date/endoflife.date using https://github.com/pdmosses/just-the-docs/blob/fix-nav-performance/_includes/nav.html produced the following profile extract:

Filename                                                    | Count |    Bytes |   Time
------------------------------------------------------------|-------|----------|-------
| just-the-docs-0.4.0.rc1/_layouts/default.html                    |   130 |  3792.04K |  5.160 |
| _includes/nav.html                                               |   130 |  1405.20K |  4.054 |
| just-the-docs-0.4.0.rc1/_includes/head.html                      |   130 |   617.82K |  0.495 |
| _layouts/product.html                                            |   127 |  1014.38K |  0.413 |
| _includes/head_custom.html                                       |   130 |   427.83K |  0.393 |
| assets/js/zzzz-search-data.json                                  |     1 |   149.31K |  0.050 |

@nathancarter has tried adding the new `nav.html` to [a site with over 300 pages](https://nathancarter.github.io/how2data/site/), and reported that it improved the build time of more than 3 minutes to about 30 seconds.

Further optimisation of navigation might be possible (e.g., using [Jekyll include caching](https://github.com/benbalter/jekyll-include-cache)), but the current optimisation should be sufficient for v0.4.0.

To test that this PR does not appear to affect the navigation panel generated by v0.3.3:

1. Clone https://github.com/just-the-docs/just-the-docs-tests.
2. Update `_config.yml` and `Gemfile` to use this PR branch.
3. Run `bundle update`.
4. Inspect the rendering of the entire collection of navigation tests.

(Many of the differences reported in the GitHub visualisation of the changes are due to shifting much of the code 2 spaces to the left, in connection with moving the first `ul` element to be close to its first item.)

Adds accessible nav elements for nested pages

Why are these changes being introduced:

* The current links to show/hide the nested pages use a visual only
  svg image with no accessible affordance provided so screenreaders
  will not be able to provide appropriate context for users as to what
  they should expect when clicking these links
* You can see the problem by running a tool like ANDI on the current
  main branch of this repository and then running it again on this
  branch. ANDI shows what a screenreader would read.
* You can also use a tool like Voiceover to hear the importance of what
  this introduces to users that use that technology. Before this change,
  Voiceover would read all of these navigation links as
  "link image just-the-docs" but with this change it will read
  `link image toggle links in <categoryName> category`

Relevant ticket(s):

* This was discussed as part of the larger WCAG compliance ticket
  https://github.com/just-the-docs/just-the-docs/issues/566

How does this address that need:

* This adds an `aria-label` to the link https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-label

Document any side effects to this change:

It appears it might be prefereable to use `aria-labelledby` whenever
possible, but from what I can tell these links are just the visual cue
of the svg with no other affordance given to users to understand what
they'll do so there is nothing to point `aria-labelledby` at.

An `svg` title was considered, but in reading more about it it seemed
like `aria-label` was more appropriate as it puts the label on the `a`
rather than the `svg` which feels more accurate.

* https://developer.mozilla.org/en-US/docs/Web/SVG/Element/title
* https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-labelledby



Co-authored-by: Matt Wang <matt@matthewwang.me>

`mermaid`: refactor config to use `mermaid_config.js` include, only require `mermaid.version` in `_config.yml` (#909)

This PR has a bit of scope creep! This PR now:

- changes the mermaid opt-in logic to only check for the existence of a `mermaid` key instead of `mermaid != false`: this resolves the follow-up in #857
- changes the behaviour of mermaid configuration
    - instead of using `mermaid_init.html` with default settings, makes the include `mermaid_config.js`
    - the include is loaded directly into the contents of `mermaid_initialize`
    - by default, it is an empty object (i.e. `{}`), triggering the defaults
- updates docs
- adds an example to the markdown kitchen sink  

It does significantly change the interface provided in #857, and I apologize for the confusion. However, given the discussion in this PR, I think it's the best move forward!

- add an extra filter to children-list to fix #854
- strengthen condition for assignment to first_level_url

Tested with Jekyll 3.9.2 and 4.2.2

Note: Jekyll 3.9.2 doesn't support `| where: "grand_parent:  nil"`

Based on #238, resolves #66.

Closes #825


Co-authored-by: Matt Wang <matt@matthewwang.me>

**Rationale**
I would like to customize the site footer content without needing to override the entire default layout. Ideally, I'd like to shorten the attribution to something like "Built with Just the Docs"

**Implementation**
Create a new `site_footer.html` include that users can override.

**Possible changes**
I debated whether or not the `<footer class="site-footer">` should be in/out of the include. I opted for placing it inside the include so I have the option of disabling the footer entirely by creating an empty site_footer file in my site.

Co-authored-by: Matt Wang <matt@matthewwang.me>
Co-authored-by: Patrick Marsceill <pmarsceill@users.noreply.github.com>

Set `child_nav_order` to `desc` to reverse the sort order for a child section.

Co-authored-by: Matt Wang <matt@matthewwang.me>

* implements search placeholder configuration

* changes implementation approach to use search_placeholder_custom

Thanks to @pdmosses for the suggestion!

* strips whitespace and HTML for both placeholder and aria-label

This PR combines (and resolves conflicts between) #448, #463, #466, #494, #495, #496, #498, and #572. 

The main aim is to facilitate use of several of the implemented features _together_, when using the fork as a remote theme. It should also simplify merging the included PRs into a future release.

The branch [combination-rec-nav](https://github.com/pdmosses/just-the-docs/tree/combination-rec-nav) adds [multi-level navigation](https://github.com/pmarsceill/just-the-docs/pull/462) and (NEW:) [sibling links](https://github.com/pmarsceill/just-the-docs/pull/394) to the branch used for this PR. It includes updated [documentation for the navigation structure](https://pdmosses.github.io/just-the-docs/docs/navigation-structure/), and reorganised and extended [navigation tests](https://pdmosses.github.io/just-the-docs/tests/navigation/). The documentation and the tests can be browsed at the (temporary) [website published from the combination-rec-nav branch](https://pdmosses.github.io/just-the-docs/

).

_Caveat:_ The changes to v0.3.3 in this PR and #462 have not yet been reviewed or approved, and may need updating before merging into a release of the theme. If you use a branch from a PR as a remote theme, there is a risk of such updates affecting your website. Moreover, these branches are likely to be deleted after they have been merged. To avoid  such problems, you could copy the branch that you want to use to your own fork of the theme.

Co-authored-by: Matt Wang <matt@matthewwang.me>

- Limit the effect of `nav_exclude: true` to the main navigation.
- Include links to excluded pages in auto-generating lists of child pages
and in breadcrumbs.
- Refactor implementation by moving assignment of `first_level_url` and `second_level_url` from `_includes/nav.html` to `_layouts/default.html`.
- Clarify the effect of `nav_exclude` in the documentation.

Correction needed for use of collections.

Co-authored-by: Kevin Lin <kevinl@cs.uw.edu>

Fixed conversion of numeric titles to strings.

Indentation adjusted

The values of `title` and `nav_order` can be numbers or strings.
Jekyll gives build failures when sorting on mixtures of different types,
so numbers and strings need to be sorted separately.

Here, numbers are sorted by their values, and come before all strings.
An omitted `nav_order` value is equivalent to the page's `title` value
(except that a numerical `title` value is treated as a string).

The case-sensitivity of string sorting is determined by `site.nav_sort`.

Pages with `nav_exclude: true` were included when sorting on `title` or `nav_order`. That could cause build failures when the type of value of the field differs from that on other pages, as reported in https://github.com/pmarsceill/just-the-docs/issues/406.

Pages with `nav_exclude: true` or no `title` are never displayed in the navigation, so removing them from `pages_list` cannot break existing sites. This change also allows the removal of some tests in the code. (The indentation of the code should now be adjusted, but that has been deferred, to restrict the size of the diff for review.)

For testing, the title of `404.html` has been changed to the number `404`,  the page `docs/untitled-test.md`  has been added, and `nav_sort_order` has been set to `case_sensitive`. Those updates give build failures with the current version of `_includes/nav.html`, but not after the suggested changes.

It will still be possible for build failures to occur due to sorting fields of *non-excluded* pages with differing types of values (e.g., `nav_order`a mixture of numbers and strings). To make the code completely safe will require relatively complicated changes,.

This improves Google Lighthouse accessibility score. As a human I agree that it was too faint.

Fixes a bug where content after headings in header elements disappeared.
Header elements are not currently used in JtD, but could make the HTML more semantic.

Now using an include parameter.
Enhanced to support Markdown with code fences.

I ran Lighthouse on just-the-docs and noticed the anchor links
were missing a "discernible name".

This adds aria-labelledby to the header element to improve these for
screenreaders.