minor #13006 [Messenger] Transports in separate packages (Nyholm)

This PR was squashed before being merged into the master branch (closes #13006).

Discussion
----------

[Messenger] Transports in separate packages

I added a note that transports will be in separate packages from 5.1.

Im not sure this is the best way to present it. Soon, new transports will be added. They should also start with a:

```
    .. code-block:: terminal

        $ composer require symfony/sqs-messenger
```

Anyhow, this is good for these 3, that is why I submitting the PR and hope for feedback.

This will fix #12998

Commits
-------

6a54402 [Messenger] Transports in separate packages

Author: javiereguiluz

messenger.rst

@@ -732,6 +732,14 @@ options.
 AMQP Transport
 ~~~~~~~~~~~~~~
 
+.. versionadded:: 5.1
+
+    The AMQP transport has moved to a separate package. Install it by running
+
+    .. code-block:: terminal
+
+        $ composer require symfony/amqp-messenger
+
 The ``amqp`` transport configuration looks like this:
 
 .. code-block:: bash
@@ -755,7 +763,7 @@ You can also configure AMQP-specific settings on your message by adding
 :class:`Symfony\\Component\\Messenger\\Transport\\AmqpExt\\AmqpStamp` to
 your Envelope::
 
-    use Symfony\Component\Messenger\Transport\AmqpExt\AmqpStamp;
+    use Symfony\Component\Messenger\Bridge\Amqp\Transport\AmqpStamp;
     // ...
 
     $attributes = [];
@@ -777,6 +785,14 @@ your Envelope::
 Doctrine Transport
 ~~~~~~~~~~~~~~~~~~
 
+.. versionadded:: 5.1
+
+    The Doctrine transport has moved to a separate package. Install it by running
+
+    .. code-block:: terminal
+
+        $ composer require symfony/doctrine-messenger
+
 The Doctrine transport can be used to store messages in a database table.
 
 .. code-block:: bash
@@ -879,6 +895,14 @@ auto_setup          Whether the table should be created
 Redis Transport
 ~~~~~~~~~~~~~~~
 
+.. versionadded:: 5.1
+
+    The Redis transport has moved to a separate package. Install it by running
+
+    .. code-block:: terminal
+
+        $ composer require symfony/redis-messenger
+
 The Redis transport uses `streams`_ to queue messages.
 
 .. code-block:: bash