Add new sections to Notification docs

[bartaz]

Done

• added JS functionality docs to Notifications
• added related components to Notifications docs
• added class reference to Notifications docs

Fixes canonical-web-and-design/vanilla-squad#1197

QA

• Open demo
• Review updated documentation:
  • https://vanilla-framework-4333.demos.haus/docs/patterns/notification
  • Check if class reference, JS functionality and related components section are all good

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 🔨.

[webteam-app]

Demo starting at https://vanilla-framework-4333.demos.haus

[bethcollins92]

LGTM with a couple of changes. Not really a blocker but the Javascript section looks a bit close to the table