Remove the outdated generic installation guide and point to
platform-specific ones instead, update contact info, protocols we
support, etc.

Removed mention that there is no release of Pleroma (there actually has
been a few the last couple of months).

Alexander Strizhakov authored 5 years ago and lain committed 5 years ago

docs for RelMe provider

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

[ci skip]

Use `systemctl` instead of `service` since it's not guaranteed the
latter will be on the system (e.g. Arch does not have it).