minor #15364 [Notifier] Adds documentation for Slack stuff added in 5.3 (0x346e3730)

This PR was squashed before being merged into the 5.3 branch.

Discussion
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[Notifier] Adds documentation for Slack stuff added in 5.3

Closes #15315,  #14694 &  #14653 by adding documentation for these additions related to the notifier.

Commits
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62997ec [Notifier] Adds documentation for Slack stuff added in 5.3

Author: javiereguiluz

_images/notifier/slack/message-reply.png

@@ -135,6 +135,120 @@ The result will be something like:
 
     The `field()` method was introduced in Symfony 5.1.
 
+Adding a header to a Slack Message
+-------------------------------------------
+
+To add a header to your message you can use the
+:class:`Symfony\\Component\\Notifier\\Bridge\\Slack\\Block\\SlackHeaderBlock` class::
+
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackDividerBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackHeaderBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackSectionBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
+    use Symfony\Component\Notifier\Message\ChatMessage;
+
+    $chatMessage = new ChatMessage('Symfony Feature');
+
+    $options = (new SlackOptions())
+        ->block((new SlackHeaderBlock('My Header')))
+        ->block((new SlackSectionBlock())->text('My message'))
+        ->block(new SlackDividerBlock())
+        ->block(
+            (new SlackSectionBlock())
+                ->field('*Max Rating*')
+                ->field('5.0')
+                ->field('*Min Rating*')
+                ->field('1.0')
+        );
+
+    // Add the custom options to the chat message and send the message
+    $chatMessage->options($options);
+
+    $chatter->send($chatMessage);
+
+The result will be something like:
+
+.. image:: /_images/notifier/slack/slack-header.png
+   :align: center
+
+.. versionadded:: 5.3
+
+    The `SlackHeaderBlock` class was introduced in Symfony 5.3.
+
+Adding a footer to a Slack Message
+-------------------------------------------
+
+To add a footer to your message you can use the
+:class:`Symfony\\Component\\Notifier\\Bridge\\Slack\\Block\\SlackContextBlock` class::
+
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackContextBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackDividerBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackSectionBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
+    use Symfony\Component\Notifier\Message\ChatMessage;
+
+    $chatMessage = new ChatMessage('Symfony Feature');
+
+    $contextBlock = (new SlackContextBlock())
+        ->text('My Context')
+        ->image('https://symfony.com/logos/symfony_white_03.png', 'Symfony Logo')
+    ;
+
+    $options = (new SlackOptions())
+        ->block((new SlackSectionBlock())->text('My message'))
+        ->block(new SlackDividerBlock())
+        ->block(
+            (new SlackSectionBlock())
+                ->field('*Max Rating*')
+                ->field('5.0')
+                ->field('*Min Rating*')
+                ->field('1.0')
+        )
+        ->block($contextBlock)
+    ;
+
+    $chatter->send($chatMessage);
+
+The result will be something like:
+
+.. image:: /_images/notifier/slack/slack-footer.png
+   :align: center
+
+.. versionadded:: 5.3
+
+    The `SlackContextBlock` class was introduced in Symfony 5.3.
+
+Sending a Slack Message as a reply
+-------------------------------------------
+
+To send your slack message as a reply in a thread you can use the
+:method:`SlackOptions::threadTs() <Symfony\\Component\\Notifier\\Bridge\\Slack\\SlackOptions::threadTs>` method::
+
+    use Symfony\Component\Notifier\Bridge\Slack\Block\SlackSectionBlock;
+    use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
+    use Symfony\Component\Notifier\Message\ChatMessage;
+
+    $chatMessage = new ChatMessage('Symfony Feature');
+
+    $options = (new SlackOptions())
+        ->block((new SlackSectionBlock())->text('My reply'))
+        ->threadTs('1621592155.003100')
+    ;
+
+    // Add the custom options to the chat message and send the message
+    $chatMessage->options($options);
+
+    $chatter->send($chatMessage);
+
+The result will be something like:
+
+.. image:: /_images/notifier/slack/message-reply.png
+   :align: center
+
+.. versionadded:: 5.3
+
+    The `threadTs()` method was introduced in Symfony 5.3.
+
 Adding Interactions to a Discord Message
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