tutorial: finish the first graphql tutorial (#1332)

* tutorial: basic crud and node api for graphql tutorial

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

@@ -0,0 +1,51 @@
+---
+id: tutorial-todo-gql-tx-mutation
+title: Transactional Mutations
+sidebar_label: Transactional Mutations
+---
+
+In this section, we continue the [GraphQL example](tutorial-todo-gql.md) by explaining how to set our GraphQL mutations
+to be transactional. That means, to automatically wrap our GraphQL mutations with a database transaction and either
+commit at the end, or rollback the transaction in case of a GraphQL error.
+
+#### Clone the code (optional)
+
+The code for this tutorial is available under [github.com/a8m/ent-graphql-example](https://github.com/a8m/ent-graphql-example),
+and tagged (using Git) in each step. If you want to skip the basic setup and start with the initial version of the GraphQL
+server, you can clone the repository and checkout `v0.1.0` as follows:
+
+```console
+git clone git@github.com:a8m/ent-graphql-example.git
+cd ent-graphql-example 
+go run ./cmd/todo/
+```
+
+## Usage
+
+The GraphQL extensions provides a handler named `entgql.Transactioner` that executes each GraphQL mutation in a
+transaction. The injected client for the resolver is a [transactional `ent.Client`](transactions.md#transactional-client).
+Hence, GraphQL resolvers that uses `ent.Client` won't need to be changed. In order to add it to our todo list application
+we follow these steps:
+
+1\. Edit the `cmd/todo/main.go` and add to the GraphQL server initialization the `entgql.Transactioner` handler as
+follows:
+
+```diff
+srv := handler.NewDefaultServer(todo.NewSchema(client))
++srv.Use(entgql.Transactioner{TxOpener: client})
+```
+
+2\. Then, in the GraphQL mutations, use the client from context as follows:
+```diff
+func (mutationResolver) CreateTodo(ctx context.Context, todo TodoInput) (*ent.Todo, error) {
++	client := ent.FromContext(ctx)
++	return client.Todo.
+-	return r.client.Todo.
+		Create().
+		SetText(todo.Text).
+		SetStatus(todo.Status).
+		SetNillablePriority(todo.Priority). // Set the "priority" field if provided.
+		SetNillableParentID(todo.Parent).   // Set the "parent_id" field if provided.
+		Save(ctx)
+}
+```