[Notifier] Add Mercure notifier documentation

Author: mtarld

notifier.rst

@@ -153,6 +153,7 @@ Firebase    ``symfony/firebase-notifier``     ``firebase://USERNAME:PASSWORD@def
 GoogleChat  ``symfony/google-chat-notifier``  ``googlechat://ACCESS_KEY:ACCESS_TOKEN@default/SPACE?thread_key=THREAD_KEY``
 LinkedIn    ``symfony/linked-in-notifier``    ``linkedin://TOKEN:USER_ID@default``
 Mattermost  ``symfony/mattermost-notifier``   ``mattermost://ACCESS_TOKEN@HOST/PATH?channel=CHANNEL``
+Mercure     ``symfony/mercure-notifier``      ``mercure://PUBLISHER_SERVICE_ID?topic=TOPIC``
 RocketChat  ``symfony/rocket-chat-notifier``  ``rocketchat://TOKEN@ENDPOINT?channel=CHANNEL``
 Slack       ``symfony/slack-notifier``        ``slack://TOKEN@default?channel=CHANNEL``
 Telegram    ``symfony/telegram-notifier``     ``telegram://TOKEN@default?channel=CHAT_ID``
@@ -170,6 +171,10 @@ Zulip       ``symfony/zulip-notifier``        ``zulip://EMAIL:TOKEN@HOST?channel
     The GoogleChat, LinkedIn, Zulip and Discord integrations were introduced in Symfony 5.2.
     The Slack DSN changed in Symfony 5.2 to use Slack Web API again same as in 5.0.
 
+.. versionadded:: 5.3
+
+    The Mercure integration was introduced in Symfony 5.3.
+
 Chatters are configured using the ``chatter_transports`` setting:
 
 .. code-block:: bash