deprecating Swift Mailer and moving all docs into one spot and moving many docs from mime to Mailer and writing it

Co-Authored-By: Wouter J <wouterj@users.noreply.github.com>
Co-Authored-By: Ricardo de Vries <ricardozegt@gmail.com>
Co-Authored-By: Antoine Makdessi <antoine.makdessi@me.com>

Author: 4 people

_build/redirection_map

@@ -417,3 +417,6 @@
 /workflow/state-machines /workflow/introduction
 /workflow/usage /workflow
 /introduction/from_flat_php_to_symfony2 /introduction/from_flat_php_to_symfony
+/email/dev_environment /mailer
+/email/spool /mailer
+/email/testing /mailer