Introduce user/op/dev structure for the documentation

Created by: hacklschorsch

This PR separates the documentation into different parts, addressed by audience: User, Administrator and Developer documentation.

In an e-Mail to me Jean-Paul wrote:

There's not much of this kind of documentation in the repo already so you'll have to invent some kind of structure but don't worry about that too much, easy to move stuff later as we get a better idea of what else we want to document.

Adding my docs in #55 to the top-level README.rst seemed not the right location. I'd rather add my bits of new knowledge into a "getting started" section of the developer documentation.

@exarkun : What do you think? Hot or not?