Don't overwrite stable release docs with unstable/release candidate docs

[huonw]

For "branches" that have a stable release, we probably shouldn't sync the docs for future release candidates on that branch, only future stable releases. E.g., we've released 2.19.0 and therefore:

• when we release 2.19.1rc123 or similar, don't sync doc changes ❌ , as that would replace the stable docs with unstable/unreleased info
• when we release 2.19.1, sync the doc changes ✅
• (when we release 2.20.0rc123 or similar, sync the doc changes ✅ because there hasn't been a stable release)

For now, we can have a convention of a human deciding to merge or not merge the doc sync PR as appropriate, but it be better for the automation to handle it automatically.

see https://chat.pantsbuild.org/t/16370494/i-assume-we-should-not-be-listing-2-17-and-2-16-as-prereleas#ad27355f-9e63-4210-839f-6a421ff0f44f

[huonw]

Potentially in direct tension with #148.