Improvements on server/v2 gRPC gateway autoregistration

[julienrbrt]

Follow up of #22941

• Create a doc.go file in server/v2/api/grpcgateway explaining the syntax supported by the catch all handlers
• Remove manual grpc grpcgateway registration from modules #23367
• Investigate if gRPC Auto-gateway can try the canonical gateway first for registered modules #23368
• currently, regexp patterns are built on the fly, but we could build these for each URI beforehand, and store them for faster processing.

ref: #22941 (comment)