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