cloudrad: Cross-library links

[dazuma]

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.