GitLab

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)

Backport of: !3485

Adjusted MediaProxyControllerTest to gracefully fail on missing dependencies. Installation docs update.
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

Update docs/installation/arch_linux_en.md, docs/installation/centos7_en.md, docs/installation/debian_based_en.md, docs/installation/debian_based_jp.md, docs/installation/gentoo_en.md files

Include explicit warning for emerge config line as it will vary more widely based on current version of postgres