[DOCS] Add info about mailer in installation doc

[arti0090]

Q	A
Branch?	1.12
License	MIT

[GSadee]

This PR seems to be a little bit outdated, probably it should be merged to the 1.11 branch in this version and be updated in 1.12 because of the PR

[GSadee]

The base of this pull-request was changed, you need fetch and reset your local branch
if you want to add new commits to this pull request. Reset before you pull, else commits
may become messed-up.

Unless you added new commits (to this branch) locally that you did not push yet,
execute git fetch origin && git reset "mailer-install" to update your local branch.

Feel free to ask for assistance when you get stuck 👍

[lchrusciel]

but we should already update this doc for 1.12 and Symfony Mailer