The TOC/breadcrumb links in "Tests" and "Test Collection 1" exhibit the bug reported in just-the-docs#492. To fix the bug, this PR needs merging with just-the-docs#494.

Manually merge the relevant changes from PR just-the-docs#494, namely:

When configured for more than one collection, v0.3.2 produces empty TOCs of children in all but the last collection, due to referencing the overwritten `pages_list` in `_layouts/default.html`. When the same parent title is used in different collections, v0.3.2 can also produce a link to the wrong collection. Moreover, v0.3.3  can also produce incorrect breadcrumb links; see issue just-the-docs#492.

- Move the breadcrumbs link variable assignments from `_layouts/default.html` to `_includes/nav.html`, and make it conditional on `page` being in the current collection.

- Correct the condition for assignment to `second_level_url`, as in just-the-docs#477.

- Change the variable used for the TOC from `children_list` to `toc_list`, move its assignment from `_layouts/default.html` to `_includes/nav.html`, and make it conditional on `page` being in the current collection.

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](#462) and (NEW:) [sibling links](#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 <[email protected]>