Added Sphinx/Read the docs/ReST documentation

This merge request adds the skeleton of what will eventually be the user documenatation, that will also contain the mathematical details of what the code does. I've used Sphinx to generate it; all of the input is in ReStructured Text (ReST), and it produces a nice html website that is much easier to navigate than a pdf. Given the correct conf.py, it can also generate a LaTeX-based pdf version of the documentation.

I'm creating this merge request now so that other developers can see the structure of the documentation and contribute.