Optimised CSS selectors.
Changed markup in `linenos-test` to inhibit line numbers appearing in YAML examples when testing code fences.

The additions to `_config.yml` go together with the changes to `code.scss`,
to facilitate adjusttments by users. See `docs/linenos-test` for the details.
The CSS code has been significantly refactored and simplified,
and seems to produce sensible results (at least on Safari and Firefox).

Fixed css .search-active.main

Improve accessibility by adding label to Anchor links.

Removed some comments and layout

Produces sensible (but not perfect) results from different kinds of highlight markup.
The CSS code needs a major refactoring to eliminate duplication and superflous lines.

Now using an include parameter.
Enhanced to support Markdown with code fences.

This should revert the changes made to index-test.md

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.

Fixed more layout details. Numbers of selectors still to be fixed.

Correected layout, order, values.

The added examples in `docs/index-test.md` extend the previous examplees of highlighting, documenting the required inout.

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

Bump stylelint-config-prettier from 8.0.1 to 8.0.2

Bumps [@primer/css](https://github.com/primer/css) from 14.3.0 to 14.4.0.
- [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/v14.3.0...v14.4.0

)

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

Bumps [stylelint-config-prettier](https://github.com/prettier/stylelint-config-prettier) from 8.0.1 to 8.0.2.
- [Release notes](https://github.com/prettier/stylelint-config-prettier/releases)
- [Commits](https://github.com/prettier/stylelint-config-prettier/compare/v8.0.1...v8.0.2

)

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

Bump version numbers

Add print styles