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
