Include release notes in Plone 6 Documentation

[stevepiercy]

It would be good to include Plone REST API release notes in Plone 6 Documentation. Options include:

• Add a page with a link to https://github.com/plone/plone.restapi/blob/main/CHANGES.rst. Quickest and easiest.
• Follow the example of Volto: https://github.com/search?q=repo%3Aplone%2Fvolto%20copyreleasenotestodocs&type=code. This option would require converting CHANGES.rst to CommonMark and renaming to CHANGES.md. This option also yields a nicer rendering on GitHub.
• Use an include directive, but that yields errors either with a double heading or a page without a title. Not a good option.