- 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.

Correction suggested by @allejo in comment https://github.com/pmarsceill/just-the-docs/pull/399#discussion_r464128713.
Also prevents losing the aria-labelledby attribute in future updates to this snippet.

Renamed doc_collections to just-the-docs.collections

- font size 16px to prevent zooming on iOS Safari
- search input is scrolled into view on iOS Safari
- box-shadow is displayed correctly

Improved documentation for search configurations

Added more search configurations
Display "no results found"

Removes children from the table of contents that
belong to a different page with the same title.

When `nav_order` is omitted, the order of nodes at each menu level (and in the auto-generated TOC) is alphabetical by `title`, instead of random.
Any nodes with a specified `nav_order` precede all nodes at that level where it is omitted.
Note that `nav_order` fields must have a uniform site-ide type: integers and strings cannot be mixed, otherwise Jekyll reports errors.
The implementation filters the ordered and unordered pages from `site.html_pages`, sorts them separately, and concatenates the resulting arrays.

(cherry picked from commit 99371e971406090a2dba7b9b69f514a8f87afce3)

(cherry picked from commit 468264941d08b00252a4f6f081bc2dff5920bffe)