Release notes in GitHub have significant-newlines, which makes our
canonical `changelog.md` content look not-great -- and the relative
links are also broken, as they have the wrong relative path.
Switch to linking to the canonical release notes on ReadTheDocs, now
that those have proper anchors.
(cherry picked from commit c1008b5b12)
- Updates `.prettierignore` for the new directory.
- Updates any reference to the API documentation directory for
markdown files to be `api_docs/` instead of `zerver/api/`.
- Removes a reference link from `docs/documentation/api.md` that
hasn't referenced anything in the text since commit 0542c60.
- Update rendering of API documentation for new directory.
These suffixes suppress some checks in the process, but still generate
and upload a tarball, push a tag, and make a Github prerelease.
`upload-release` already understands that anything with a suffix never
becomes the "latest" release.
