Based on #238, resolves #66.

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

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

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>

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.

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.)

Clarifications

Adjusted the documentation to explain how mixtures of numbers and strings are treated by `nav_order`.

Makes the TOC at the top of the page collapsible.
Adds an explanation of the markup used to produce a collapsible TOC.
Adds a comment about the possibility of using an unordered list for the TOC,
and about using TOC only once per page.

closes #266

Deleted a spurious line from a previous version.

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

Added a configuration option to determine whether the sort order is case-sensitive.
The default is case-insensitive.

To test:
- open `/just-the-docs/docs/utilities/` in the browser,
  and check that the navigation links in `Utilities` are sorted alphabetically;
- in `docs/utilities/layout.md', change the preamble to `title: layout`,
  and check that the  links in `Utilities` are still sorted alphabetically;
- add `nav_sort: case_sensitive` in the configuration file,
  and check that the link to `layout` is now listed last under `Utilities`.

Added documentation of default nav order.

This reverts commit 8ac545a5.

This reverts commit 89f79138.

This reverts commit 8ac545a5.

The current default is that when a grandparent is selected, the links to its
children *and all its grandchildren* are displayed in the navigation menu.
Setting `grandchildren_branch: true` in `_config.yml` delays the display of links
to grandchildren until their parent is selected.

Using `index.md` as name for the parent page within a section allows lazy developers
to skip defining `permalink`. This is already working and this PR just updates the documentation.