Update Code snippet docs

[bartaz]

Done

Adds class reference and JavaScript sections to code snippet.

Fixes canonical-web-and-design/vanilla-squad#1198

QA

• Review updated documentation:
  • Class reference https://vanilla-framework-4415.demos.haus/docs/base/code#class-reference
  • JavaScript functionality https://vanilla-framework-4415.demos.haus/docs/base/code#javascript-functionality

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.

[webteam-app]

Demo starting at https://vanilla-framework-4415.demos.haus

[bethcollins92]

LGTM 👍🏼