Feature/doc collections

Adjust dl layout

Cosmetic

Works for description lists with multiple `dt` and `dd` elements.

Bump lodash from 4.17.15 to 4.17.19

Bump @primer/css from 14.4.0 to 15.0.0

Fix https

Bumps [@primer/css](https://github.com/primer/css) from 14.4.0 to 15.0.0.
- [Release notes](https://github.com/primer/css/releases)
- [Changelog](https://github.com/primer/css/blob/master/CHANGELOG.md)
- [Commits](https://github.com/primer/css/compare/v14.4.0...v15.0.0

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.15 to 4.17.19.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](https://github.com/lodash/lodash/compare/4.17.15...4.17.19

)

Signed-off-by: dependabot[bot] <support@github.com>

Renamed doc_collections to just-the-docs.collections

Fixed css .search-active.main

Improve accessibility by adding label to Anchor links.

I ran Lighthouse on just-the-docs and noticed the anchor links
were missing a "discernible name".

This adds aria-labelledby to the header element to improve these for
screenreaders.

Remove collapsible TOC on nav doc

Pdmosses collapsible toc

Merge branch 'collapsible-toc' of git://github.com/pdmosses/just-the-docs into pdmosses-collapsible-toc

Bump @primer/css from 14.3.0 to 14.4.0

Bump stylelint from 13.3.3 to 13.6.1