feat: markdown in email body for mailing and transactional

templates/admin/system/email_text/_form.html.twig

@@ -4,6 +4,8 @@
     {{ form_row(form.headline) }}
     {{ form_row(form.body) }}
 
+    {{ include('_partials/_markdown_hint.html.twig') }}
+
     <div class="bg-gray-100 rounded-md p-4 text-sm">
         <div class="font-bold pb-2">
             Verfügbare Platzhalter