cloudrad: Better experience for toplevel module pages that are otherwise blank

Example: https://cloud.google.com/ruby/docs/reference/google-cloud-monitoring-v3/latest/Google

We should be able to provide some special-case content for such modules. (Note, of course, that the canonical rubydoc.info docs also show a pretty much blank page: https://rubydoc.info/gems/google-cloud-monitoring-v3/Google)