I understand that this change is quite unreadable and hard to review, sorry for forgetting to do atomic commits. This patch does not change too much content wise, it just
* Gets everything from gitlab wiki
* Removes some specific gitlab hacks
* Formats all documentation file names to be in snake case so they look the same way as our code does

[ci skip]

Use `systemctl` instead of `service` since it's not guaranteed the
latter will be on the system (e.g. Arch does not have it).

password parameter was missing

* How to use MIX_ENV

* How to create a user

* Link to certbot's site with the webform which advises how to obtain
  a certificate based on web server/OS.

Also remove the erlang packages, they're specified in the Debian guide.

$ mix phx.serve
** (Mix) The task "phx.serve" could not be found. Did you mean "phx.server"?

$ mix phx.server
[info] Running Pleroma.Web.Endpoint with Cowboy using http://0.0.0.0:4000