cloudrad: Cross-library links

Cross-reference links sometimes span libraries. Common cases include:

* References in wrapper libraries that reference types in gapic libraries.
* References to protobuf core classes like `::Google::Protobuf::Map` (usually common types such as `::Google::Protobuf::Timestamp` are already generated in proto_docs). We should include the google-protobuf gem in cloudrad and link to the appropriate definitions there
* Similar for other common gems such as gapic-common and google-cloud-core.
* References to Ruby core classes that could link to ruby-doc.org

We should have enough control over the overall generation and maintenance of these docs that we can provide cross-reference links for those cases.