documentation reorganizing

[Krastanov]

Currently we have two parallel sections in the documentation which have pretty bad overlap: "Tutorial" and "Topical Guides". For instance, both have a "basics" sections. We should clean this up so that the flow of the documentation is clearer.

Following the Divio system is not a bad idea either https://documentation.divio.com/ (I think that was a goal, but the current execution of it is pretty confusing with no clear distinction between "tutorial" and "guide").

[filchristou]

What I find most important is to have better API documentation. Even though functions are described in the docs, most of the times ? <func> brings nothing..