For contributions to the docs you could open a “issues-only” repo on GitHub.
Or a separate docs repo. That would also allow you to have branches/tags to match the current code base. Folks could add docs for new features before they’re live and there’s no need to worry about which docs are live. When the code updates, the docs do too.
I’d help out here and there as I come across things but the barrier to entry is a little blurred.
