Add Symfony Notifier integration

Integrated Symfony Notifier into the project by adding it to `composer.json` and creating a configuration file `notifier.yaml`. Updated `symfony.lock` to include the recipe configuration for Notifier. Minor documentation formatting issues were also fixed in `index.rst`.

config/packages/notifier.yaml

@@ -0,0 +1,12 @@
+framework:
+    notifier:
+        texter_transports:
+            twilio: '%env(resolve:SHORT_MESSAGE_DSN)%'
+        channel_policy:
+            # use chat/slack, chat/telegram, sms/twilio or sms/nexmo
+            urgent: ['email']
+            high: ['email']
+            medium: ['email']
+            low: ['email']
+        admin_recipients:
+            - { email: admin@example.com }