Update docs for feat: add GRPC unary call service to our GreptimeDB

[sunng87]

A document change request is generated from
GreptimeTeam/greptimedb#1196

[MichaelScofield]

should also add back grpc reflection contents. related:

• docs: add "How to write gRPC SDK for GreptimeDB" #201
• docs: remove outdated grpc contents #199
• docs: remove outdated contents about our gRPC service #175