Move query plan mode setting from OTP installation guide and try
to explain what it does.

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.

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).

Since python doesn't have a way to lock deps for a particlar project
by default, I didn't bother with it. This resulted in mkdocs updating at
some point, bringing a breaking change to how tabs are declared and
broken tabs on docs-develop.pleroma.social. I've learned my lesson
and locked deps with pipenv in docs!5. This MR updates Pleroma
docs to use the new tab style, fortunately my editor did most of it.

Closes #2045

This reverts merge request !2763

* 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

Needs to be backported to stable.

Related: pleroma-support#29

Needs to be backported to stable.

Related: pleroma/pleroma-support#29

* I removed the Update section from the OTP install and added it to updating.md
* I also added a link to the updating.md from the OTP install page
* I added a Questions section to the OTP install, similar to the debian_based_en.md
* Restructured the updating.md a bit

I would very much also like to link to the changelog, but I don't know how to do it because I can't find the page in the docs?

in the config generator