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