* 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
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 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
