RepoKitteh-based PR checklists

We can have RK automatically label any change to source/ or api/ without a change to current.rst with a release-notes blocker, requiring manual confirmation from maintainers that the release notes step has been satisfied. Similarly this could work for documentation checks, i.e. validate that at least some change has happened in docs/.

I'd propose that we take the existing ownerscheck.star and add a path expression that suppresses a given check. That should be all that is needed for release notes or docs.

@envoyproxy/maintainers are there other common review checklist items that RK could help with? CC @itayd