Add custom header and footer include files

Bump stylelint-config-primer from 9.0.0 to 9.2.1

Limit the effect of `nav_exclude` to the main navigation

Bump stylelint from 13.6.1 to 13.7.2

Bump @primer/css from 15.1.0 to 15.2.0

Bump prettier from 2.1.1 to 2.1.2

Update normalize.scss

Update code.scss

Fix list alignment

Bumps [stylelint-config-primer](https://github.com/primer/stylelint-config-primer) from 9.0.0 to 9.2.1.
- [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/v9.0.0...v9.2.1

)

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

Resolve #420

- Remove `margin-top` for the first child in various list items, to improve alignment.
- Add examples of lists to `docs/tests/styling`.

Fix #417

- Avoid appearance of dark border around visited code links in Safari.

Fix #387
- Update `_sass/vendor/normalize.scss/normalize.scss` to v8.0.1.
- Remove outdated `_sass/vendor/normalize.scss/package.json`.
- Update `_sass/vendor/normalize.scss/README.md`.

Add a test for explicit exclusion of an untitled page.

- 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.

Improve folder names.
Correct classification of excluded test pages.

Make the titles of test pages more suggestive.
Refactor the documentation of the tests.

Bumps [stylelint](https://github.com/stylelint/stylelint) from 13.6.1 to 13.7.2.
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stylelint/stylelint/compare/13.6.1...13.7.2

)

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

Bumps [@primer/css](https://github.com/primer/css) from 15.1.0 to 15.2.0.
- [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/v15.1.0...v15.2.0

)

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

Bumps [prettier](https://github.com/prettier/prettier) from 2.1.1 to 2.1.2.
- [Release notes](https://github.com/prettier/prettier/releases)
- [Changelog](https://github.com/prettier/prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/prettier/compare/2.1.1...2.1.2

)

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

v0.3.2

Bump prettier from 2.0.5 to 2.1.1