Change button copy on theme preview

Search config

Added documentation for `search_tokenizer_separator`.

[Security] Bump mixin-deep from 1.3.1 to 1.3.2

Bump @primer/css from 12.6.0 to 12.7.0

Allow for custom overrides by the user

Fix trailing comma bug in search-data.json

Improvement/site logo

Update nav.html to eliminate generation of undisplayed elements

Improve heading anchors

Recommend using index.md as parent page for sections

Introduces the possibility for a summary in the table of contents

Bumps [@primer/css](https://github.com/primer/css) from 12.6.0 to 12.7.0.
- [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/v12.6.0...v12.7.0

)

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

The default is for hyphens to separate tokens in search terms: `gem-based` is equivalent to `gem based`.

This adds `search_tokenizer_separator` as a site configuation option, to support search for hyphenated words.

This reverts commit 13ef1850.

This reverts commit 0cfc0f03.

This reverts commit 8ac545a5.

This reverts commit c282e4b0.

This reverts commit 5d58a663.

This reverts commit 89f79138.

This reverts commit 209ac7be.

Bumps [mixin-deep](https://github.com/jonschlinkert/mixin-deep) from 1.3.1 to 1.3.2. **This update includes a security fix.**
- [Release notes](https://github.com/jonschlinkert/mixin-deep/releases)
- [Commits](https://github.com/jonschlinkert/mixin-deep/compare/1.3.1...1.3.2

)

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

Restrict navigation to display only direct children.

This reverts commit 8ac545a5.

This reverts commit c282e4b0.

Separating new PR from #188, since indpendent.

The current default is that when a grandparent is selected, the links to its
children *and all its grandchildren* are displayed in the navigation menu.
Setting `grandchildren_branch: true` in `_config.yml` delays the display of links
to grandchildren until their parent is selected.

Automatically replaces the title with a logo

A `navigation-list-child-list` is not displayed unless the enclosing `navigation-list-item` is `active`, so it needs to be generated only when `page.url == node.url or page.parent == node.title or page.grand_parent == node.title`.

(cherry picked from commit 97d1c663ef566ffe20fb1ec217fd2d8c1a8a2382)

(cherry picked from commit 468bf8368783309e69fdcaa91f7eb5b1e05d96aa)