Bump @primer/css from 12.6.0 to 12.7.0

Bump stylelint-config-primer from 7.0.1 to 8.0.0

[Security] Bump mixin-deep from 1.3.1 to 1.3.2

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>

Bumps [stylelint-config-primer](https://github.com/primer/stylelint-config-primer) from 7.0.1 to 8.0.0.
- [Release notes](https://github.com/primer/stylelint-config-primer/releases)
- [Changelog](https://github.com/primer/stylelint-config-primer/blob/master/CHANGELOG.md)
- [Commits](https://github.com/primer/stylelint-config-primer/compare/v7.0.1...v8.0.0

)

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

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>

Fix blank_lines var to blanklines in config.yml

This reverts commit 7220197a.

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>

Mathjax config options added.
`compress_html` config option changed,
Mathjax script loaded by head when `page.mathjax` is truthy.
Documentation and test page added.
See also issue #198.