feature #15597 [Notifier] Add Documentation for Events (ismail1432)

This PR was merged into the 5.4 branch.

Discussion
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[Notifier] Add Documentation for Events

I would like to add documentation for the events available in the Notifier, see #15589

I don't know if I have the good approach (create a specific page like for [console events](https://symfony.com/doc/current/components/console/events.html))

Commits
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98270d1 init doc

Author: OskarStark

notifier/events.rst

@@ -0,0 +1,74 @@
+.. index::
+    single: Notifier; Events
+
+Using Events
+============
+
+The class:``...\\..\\Transport`` of the Notifier component allows you to optionally hook
+into the lifecycle via events.
+
+The ``MessageEvent::class`` Event
+---------------------------------
+
+**Typical Purposes**: Doing something before the message is send (like logging
+which message is going to be send, or displaying something about the event
+to be executed.
+
+Just before send the message, the event class ``MessageEvent`` is
+dispatched. Listeners receive a
+:class:`Symfony\\Component\\Notifier\\Event\\MessageEvent` event::
+
+    use Symfony\Component\Notifier\Event\MessageEvent;
+
+    $dispatcher->addListener(MessageEvent::class, function (MessageEvent $event) {
+        // gets the message instance
+        $message = $event->getMessage();
+
+        // log something
+        $this->logger(sprintf('Message with subject: %s will be send to %s, $message->getSubject(), $message->getRecipientId()'));
+    });
+
+The ``FailedMessageEvent`` Event
+--------------------------------
+
+**Typical Purposes**: Doing something before the exception is thrown (Retry to send the message or log additional information).
+
+Whenever an exception is thrown while sending the message, the event class ``FailedMessageEvent`` is
+dispatched. A listener can do anything useful before the exception is thrown.
+
+Listeners receive a
+:class:`Symfony\\Component\\Notifier\\Event\\FailedMessageEvent` event::
+
+    use Symfony\Component\Notifier\Event\FailedMessageEvent;
+
+    $dispatcher->addListener(FailedMessageEvent::class, function (FailedMessageEvent $event) {
+        // gets the message instance
+        $message = $event->getMessage();
+
+        // gets the error instance
+        $error = $event->getError();
+
+        // log something
+        $this->logger(sprintf('The message with subject: %s has not been sent successfully. The error is: %s, $message->getSubject(), $error->getMessage()'));
+    });
+
+
+The ``SentMessageEvent`` Event
+------------------------------
+
+**Typical Purposes**: To perform some action when the message is successfully sent (like retrieve the id returned
+when the message is sent).
+
+After the message has been successfully sent, the event class ``SentMessageEvent`` is
+dispatched. Listeners receive a
+:class:`Symfony\\Component\\Notifier\\Event\\SentMessageEvent` event::
+
+    use Symfony\Component\Notifier\Event\SentMessageEvent;
+
+    $dispatcher->addListener(SentMessageEvent::class, function (SentMessageEvent $event) {
+        // gets the message instance
+        $message = $event->getOriginalMessage();
+
+        // log something
+        $this->logger(sprintf('The message has been successfully sent and have id: %s, $message->getMessageId()'));
+    });