`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!

Closes #899 (or at least, the immediate concern).

Fix #734.

- Remove `white-space: nowrap;` in `_sass/content.scss`.
- Add an example testing wrapping in`docs/index-test.md`.

Fixes a problem where multiple labels right after a heading are vertically misaligned.

Closes #751.

Based on #238, resolves #66.

Closes #825


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

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

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

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

Looking at how the variables are defined in _variables.scss, with various dependencies between them aimed at ensuring a consistent color scheme, one might expect that redefining a given variable would affect the remaining styles that depend on it.

This is not the case, however, due to the order in which the files are processed. This PR edits the documentation to mention the non-propagating behavior of redefined variables, to better guide users that wish to customize the site using custom themes, and call their attention to the need to redefine the dependency relations as well.

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

Fix #517

Users expect nested task lists to have the same indentation as other lists.
Fix the styling in `_sass/content.scss` to do that.
Add examples in `docs/index-test.md` to test.

As is, this is invalid and will not cause the TOC to be rendered. The collapsing example is fine.

Co-authored-by: Matt Wang <matt@matthewwang.me>
Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

These docs outline some of the custom includes that are provided by the theme and can be modified by the user.

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

* Show how to break an ol

Close #750

The theme uses CSS counters for ordered lists.
So to continue the numbering of an ordered list after a break,
use `style="counter-reset:none"` instead of `start="42"`.
Add the example from #750 to `docs/index-test.md` to test.

* Show how to start from 42

Close #750

The theme uses CSS counters for ordered lists.
To start the numbering of an ordered list with 42,
use `{:style="counter-reset:step-counter 41"}` instead of `start="42"`.

Add the example from #750 to `docs/index-test.md` to test.

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>

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

Changed small-image to match large image and fix reference

Renaming the large image and changed the index test references to display these images

2019 replaced by 2020 in line 94.

Resolve #420

- Remove `margin-top` for the first child in various list items, to improve alignment.
- Add examples of lists to `docs/tests/styling`.

Add a test for explicit exclusion of an untitled page.

- 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.

Improve folder names.
Correct classification of excluded test pages.

Make the titles of test pages more suggestive.
Refactor the documentation of the tests.

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

doc for turning on kramdown linenos globally corrected/

Reinstated the collapsible TOC at the top, to support the reference to it right at the end of the file. (The `TOC` feature can only be used once per page, so this is the only way of illustrating the rendering of the collapsible TOC in the docs.)