Feature: Allow unlimited multi-level navigation (#1431)

* Allow unlimited multi-level navigation

This PR supersedes #462.

The only user-level difference from #462 is that disambiguation of parent pages has to use either `grand_parent` or `ancestor` titles: the somewhat unnatural `section_id` and `in_section` fields are not supported.

The implementation has been significantly simplified by the changes introduced in v0.7.0 of the theme.

* Detect cyclic parenthood

A page should not have a parent or ancestor with the same title. If it does, the location of the repeated link is marked by ∞, to facilitate debugging the navigation (and an unbounded loop leading to a build exception is avoided).

* Add nav_error_report warning in main navigation

When activated by `nav_error_report: true` in `_config.yml`, displays warnings about pages with the same title as their parent page or an ancestral page.

* Cache site-nav with links to all pages

The extra cached site-nav is used for determining breadcrumbs and children navigation, which may involve pages that are excluded from the main navigation.

* Replace code for determining children by inclusion of components/nav/children.html

* Update CHANGELOG.md

---------

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

CHANGELOG.md

@@ -17,12 +17,16 @@ This website is built from the `HEAD` of the `main` branch of the theme reposito
 
 Code changes to `main` that are *not* in the latest release:
 
-- N/A
+### New Features
+
+- Added: Allow unlimited multi-level navigation by [@pdmosses] in [#1431]
 
 Docs changes made since the latest release:
 
 - N/A
 
+[#1431]: https://github.com/just-the-docs/just-the-docs/pull/1431
+
 ## Release v0.9.0
 
 Hi folks! This minor release adds a `nav_enabled` set of variables to enable/disable the navigation at a site, layout, and page level --- and uses that to add search and auxilary links to the `minimal` layout. In addition, it fixes `search-data.json` corruption with default layouts and some minor CSS/SCSS issues.