Beta/Stable v5 tasks

Tracking for before we go stable with v5—we'll need to update robots.txt.

TODO:

• Redirect v5.getbootstrap.com pages or add a callout at the top of each page
• Make sure there are no page overrides on v4-dev docs; we should be careful after we move v5 docs to the main site and when we release the next 4.x version
• Unmark RTL as experimental?

Beta 1:

• Revert 723cd8d1 (not blindly) - Done in #32255
• Revert #32099 - Done in #32255
• Remove unneeded docs redirects? We may skip this

Stable:

• Revert fc74a0d9 (when we switch to the latest npm tag)
• Revert #32194