Add Notifier component documentation

This merges the work of symfony#12678, symfony#12846, symfony#12963 and finishes the main Notifier
guide.

Author: wouterj

_build/redirection_map

@@ -479,3 +479,4 @@
 /components/translation/usage /translation
 /components/translation/custom_formats https://github.com/symfony/translation
 /components/translation/custom_message_formatter https://github.com/symfony/translation
+/components/notifier https://github.com/symfony/notifier

components/notifier.rst

@@ -47,6 +47,7 @@ Topics
     mercure
     messenger
     migration
+    notifier
     performance
     profiler
     routing

index.rst

@@ -12,6 +12,9 @@ integration, CSS inlining, file attachments and a lot more. Get them installed w
 
     $ composer require symfony/mailer
 
+
+.. _mailer-transport-setup:
+
 Transport Setup
 ---------------