This release focuses on enhanced API tokens, repository ACL APIs (which
will be backported to 4.0), some performance improvements, and other bug
fixes.

THis change adds the release notes, future-proofs some Sphinx
configuration and references for 5.x releases, and tweaks the docs on
the `invalidate-api-tokens` management command to let us link and to
show the version in which this was added.

Testing Done:
Built the release notes. Checked for build errors, bad links, and spelling
errors.

Reviewed at https://reviews.reviewboard.org/r/12542/