feature #11633 The Mailer Documentation (weaverryan)

This PR was merged into the 4.3 branch.

Discussion
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The Mailer Documentation

o/ friends!

Say hello to the Mailer documentation (#11626 does the component).

A lot of the docs were moved from the mime component, and pretty much everything here was tested. Reviews, questions and comments are warmly welcomed.

Note: the first commit was just to "deprecate" the Swift Mailer docs and copy the `email/*` docs into the one, deprecated Swift Mailer document (`email.rst`). Mailer lives in `mailer.rst`.

Cheers!

Commits
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c2e92a5 deprecating Swift Mailer and moving all docs into one spot and moving many docs from mime to Mailer and writing it

Author: weaverryan

_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