This commit starts restructuring the documentation and updating it to be
compliant with 0.12.x+ releases.
The main change is that the documentation has been rewritten for the
ground up, and hopefully simplified.
The documentation has been split into an official documentation for
running headscale as a binary under Linux with SystemD and a "community"
provided documentation for Docker.
This should make the two documents a lot easier to read and follow than
the mishmash document we had.
