AFAIK OTP releases are the recomended way of installing, but
* People seem unaware of that and use from source installations because they use the guide with the name of their distro
* People don't know what OTP releases are or what it means
I added a warning on all installation-from-source guides and added the same explanation on the two OTP pages (the miigration to OTP and installing OTP)
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
