minor #12925 [Notifier] Added a placeholder article for the component docs (javiereguiluz)

This PR was squashed before being merged into the 5.0 branch (closes #12925).

Discussion
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[Notifier] Added a placeholder article for the component docs

Notifier doesn't have docs yet. We don't link to them from symfony.com ... but we link to them from the README of the code repo. So, let's add a placeholder article to avoid 404 errors.

Commits
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3095dff [Notifier] Added a placeholder article for the component docs

Author: javiereguiluz

components/notifier.rst

@@ -0,0 +1,33 @@
+.. index::
+   single: Notifier
+   single: Notifications
+   single: Components; Notifier
+
+The Notifier Component
+======================
+
+    The Notifier component sends notifications via one or more channels
+    (email, SMS, Slack, ...).
+
+.. versionadded:: 5.0
+
+    The String component was introduced in Symfony 5.0 as an
+    :doc:`experimental feature </contributing/code/experimental>`.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/notifier
+
+.. include:: /components/require_autoload.rst.inc
+
+
+Usage
+-----
+
+.. caution::
+
+	We're still working on the docs of this component. Check this page again
+	in a few days.