Add accessibility accordion notes

[bethcollins92]

Done

• Added accessibility notes - google doc is here and has been QA'd by @sowasred2012 and @wgx

Fixes #4037

QA

• Open demo
• Check the accessibility docs are the same as the doc

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-4233.demos.haus

[bethcollins92]

@sowasred2012 could you double check and approve this please? I've just copied the google doc you've already reviewed to the Accordion section. Thanks!

[sowasred2012]

LGTM, just a couple of minor tweaks!