docs: Adapt gRPC docs to GAX v4 and repo changes
docs: Adapt gRPC docs to GAX v4 and repo changes

Even though we depend on Grpc.Net.Client, it makes sense to only
generate docs for Grpc.Core.Api and Grpc.Auth, as those are the
cross-implementation packages. (The dependency on Grpc.Net.Client is
more for convenience than anything else.)

googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet