* I cretaed a folder 'development'
* I split up the file dev.md into three parts and moved it to this folder
  * index.md
  * authentication_authorization.md
  * mrf.md
* I also moved ap_extensions.md
* I created a new file setting_up_pleroma_dev.md

It's libmagic-dev in both Ubuntu and Debian.

Reported in private by NaiJi.

While taking a final look at instance.gen before releasing I noticed
that the release_env task outputs messages in broken english. Upon
further inspection it seems to have even more severe issues which, in
my opinion, warrant it's at least temporary removal:
- We do not explain what it actually does, anywhere. Neither the task
 docs nor instance.gen, nor installation instructions.
- It does not respect FHS on OTP releases (uses /opt/pleroma/config even
 though we store the config in /etc/pleroma/config.exs).
- It doesn't work on OTP releases, which is the main reason it exists.
Neither systemd nor openrc service files for OTP include it.
- It is not mentioned in install guides other than the ones for Debian
and OTP releases.

Added ffmpeg/imagemagick checks to launch checks (if media preview proxy is enabled). Added documentation on installing optional media / graphics packages (imagemagick, ffmpeg, exiftool).

* I added an include and use this include for the installation guides that already had this section
* I added the "Further reading" section as well as te "Questions" section to the English guides that didn't have it yet
* I added a first point "How Federation Works/Why is my Federated Timeline empty?" to link to lains blogpost about this because we still get this question a lot in the #pleroma support channel
* I reordered the list a bit

Added the erlang-ssh package to the dependency list and the installation
command instructions. The project wouldn't build otherwise.

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