user documentation for email submission server

#30608 (closed) is technically done: users can change their mail password and configure their mail clients to send mail "as torproject.org".

but i doubt people are really aware of this, nor are in a position to do so. i suspect, for example, many people are not familiar with the db.torproject.org interface.

the task here is to document, from A to Z, how to setup a "mail account" with Thunderbird (mostly done), Gmail, Mac's Mail.app, Outlook, etc.

I started building some documentation in https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/submission#tutorial but it could use some work, and especially screenshots.

@gaba, do you want to give a hand on this?