doc/md: various improvements to gRPC tutorial (#2977)

doc/md/tutorial-grpc-generating-proto.md

@@ -7,7 +7,7 @@ As Ent and Protobuf schemas are not identical, we must supply some annotations o
 
 The first thing we need to do is to add an `entproto.Message()` annotation. This is our opt-in to Protobuf schema generation, we don't necessarily want to generate proto messages or gRPC service definitions from *all* of our schema entities, and this annotation gives us that control. To add it, append to `ent/schema/user.go`:
 
-```go
+```go title="ent/schema/user.go"
 func (User) Annotations() []schema.Annotation {
 	return []schema.Annotation{
 		entproto.Message(),
@@ -17,7 +17,7 @@ func (User) Annotations() []schema.Annotation {
 
 Next, we need to annotate each field and assign it a field number. Recall that when [defining a protobuf message type](https://developers.google.com/protocol-buffers/docs/proto3#simple), each field must be assigned a unique number.  To do that, we add an `entproto.Field` annotation on each field. Update the `Fields` in `ent/schema/user.go`:
 
-```go
+```go title="ent/schema/user.go"
 // Fields of the User.
 func (User) Fields() []ent.Field {
 	return []ent.Field{
@@ -37,7 +37,7 @@ func (User) Fields() []ent.Field {
 
 Notice that we did not start our field numbers from 1, this is because `ent` implicitly creates the `ID` field for the entity, and that field is automatically assigned the number 1.  We can now generate our protobuf message type definitions. To do that, we will add to `ent/generate.go` a `go:generate` directive that invokes the `entproto` command-line tool. It should now look like this:
 
-```go
+```go title="ent/generate.go"
 package ent
 
 //go:generate go run -mod=mod entgo.io/ent/cmd/ent generate ./schema
@@ -61,7 +61,7 @@ ent/proto
 
 Two files were created. Let's look at their contents:
 
-```protobuf
+```protobuf title="ent/proto/entpb/entpb.proto"
 // Code generated by entproto. DO NOT EDIT.
 syntax = "proto3";
 
@@ -80,7 +80,7 @@ message User {
 
 Nice! A new `.proto` file containing a message type definition that maps to our `User` schema was created!
 
-```go
+```go title="ent/proto/entpb/generate.go"
 package entpb
 //go:generate protoc -I=.. --go_out=.. --go-grpc_out=.. --go_opt=paths=source_relative --go-grpc_opt=paths=source_relative --entgrpc_out=.. --entgrpc_opt=paths=source_relative,schema_path=../../schema entpb/entpb.proto
 ```