Add user documentation

[Fohlen]

I think we are at a point where we came to census to use readthedocs style user documentation.

This will mean:

• user guides, e.g: installing, playing, editing (...) will go to this repo
  • also server guides
  • some basic modding information
• developer information remains in the inexor-core wiki

Here are a few tickets that refer to this overall ticket, and which I closed

• 461

• 262

• 443

• 90

[MartinMuzatko]

Do you want to create an extra repo for that?