Add heading styles for side navigation

[bartaz]

Done

Adds new p-side-navigation__heading class (with a --linked variant) to be used in docs to provide headings for side navigation item groups.

Drive by: Fixes an issue with too small embedded examples for side nav components

Fixes #4485

QA

• Open demo
• Check if side navigation headings look as expected
• Review updated documentation:
  • Make sure it works in all variants:
  • https://vanilla-framework-4487.demos.haus/docs/examples/patterns/side-navigation/default
  • https://vanilla-framework-4487.demos.haus/docs/examples/patterns/side-navigation/dark

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix relesase (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.
• Documentation side navigation should be updated with the relevant labels.

Screenshots

[webteam-app]

Demo starting at https://vanilla-framework-4487.demos.haus

[andesol]

LGTM!