Docs: Remove warning when browsing away in React

[tmeasday]

Closes #20731

What I did

Put the cleanup() in a timeout, as advised on the react issue.

Added a story so we can check teardown is still working (only react right now)

How to test

1. Open a docs page
2. Browse away
3. Check it doesn't log warning
4. Browse away from teardown story in docs + story mode, check it logs 'unmounted'.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]