Move validphys guide over to sphinx

Created by: Zaharid

The people keep complaining that now that we have two places where we can put the documentation, nobody knows where to add it or where to look for it or where to add more documentation.

We agreed to move to sphinx and in fact we have it working. However it is sort of empty. We should move the content on the guide over to there, and give it some more structure. The high level idea of how should it look like is described in #287 (closed).