doc/tutorial: add title with file names to code blocks (#2430)

doc/md/tutorial-todo-gql-mutation-input.md

@@ -26,7 +26,7 @@ The Ent framework accepts external templates that can extend or override the def
 code generator. In the template below, we generate 2 input types (`CreateTodoInput` and `UpdateTodoInput`) for the
 GraphQL mutations, and add additional methods on the different builders to accept these objects as an input type.
 
-```gotemplate
+```gotemplate title="ent/template/mutation_input.tmpl"
 {{ range $n := $.Nodes }}
     {{ $input := print "Create" $n.Name "Input" }}
     // {{ $input }} represents a mutation input for creating {{ plural $n.Name | lower }}.
@@ -113,7 +113,7 @@ go generate ./...
 
 You may have noticed that Ent generated a new file `ent/mutation_input.go` with the following content:
 
-```go
+```go title="ent/template/mutation_input.go"
 // Code generated by entc, DO NOT EDIT.
 
 package ent
@@ -160,7 +160,7 @@ func (i *UpdateTodoInput) Mutate(m *TodoMutation) {
 The new generated Go types are the GraphQL mutation types. Let's define them manually in the GraphQL schema and `gqlgen`
 will map them automatically.
 
-```graphql
+```graphql title="todo.graphql"
 # Define an input type for the mutation below.
 # https://graphql.org/learn/schema/#input-types
 #
@@ -207,7 +207,7 @@ go generate ./...
 
 The result is as follows:
 
-```go
+```go title="todo.resolvers.go"
 func (r *mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoInput) (*ent.Todo, error) {
 	panic(fmt.Errorf("not implemented"))
 }
@@ -225,7 +225,7 @@ func (r *mutationResolver) UpdateTodos(ctx context.Context, ids []int, input ent
 
 The `Set<F>` calls in the `CreateTodo` resolver are replaced with one call named `SetInput`:
 
-```diff
+```diff title="todo.resolvers.go"
 func (r *mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoInput) (*ent.Todo, error) {
 	return ent.FromContext(ctx).Todo.
 		Create().
@@ -240,7 +240,7 @@ func (r *mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoI
 
 The rest of the resolvers (`UpdateTodo` and `UpdateTodos`) will be implemented as follows:
 
-```diff
+```diff title="todo.resolvers.go"
 func (r *mutationResolver) CreateTodo(ctx context.Context, input ent.CreateTodoInput) (*ent.Todo, error) {
 	return ent.FromContext(ctx).Todo.Create().SetInput(input).Save(ctx)
 }