Document the Mailer component

[javiereguiluz]

In 4.3 we added two closely related components: Mime and Mailer.

We have the full docs of Mime: https://symfony.com/doc/master/components/mime.html

But Mailer must be documented: https://symfony.com/doc/master/components/mailer.html

I tried to use this presentation from Fabien to document the Mailer, but I couldn't because I don't fully understand it: https://speakerdeck.com/fabpot/mailer?slide=35

[cvergne]

Hello,
Is there any way to help ? Is there already a draft ? :)

[harikt]

I have made an attempt. See #11626 .

[bocharsky-bw]

More related changes in #11633