Setup documentation and deploy it about scikit-matter

[rosecers]

++ readthedocs, highly prefer that medium to github pages

[Luthaf]

There should be no difference in the generated doc between github pages & readthedocs, since both would be based on sphinx.

Do you have any specific issues with github pages?

I tend to prefer github pages because I had some issues with readthedoc compilation environment for a C++ based project (not really applicable here, but we still have more control over doc deployment with github pages). The big advantage I can see for readthedocs is the automatic generation of project.readthedocs.io/latest/project.readthedocs.io/v0.1.0/project.readthedocs.io/v0.2.0 (i.e. having doc online for multiple version of the project). This can be acheived on github pages but needs a bit more work.