Documentations

[LecrisUT]

TODO:

• Add RTD/sphinx framework
• Enable RTD build withPR support. @pboettch this you need to add on https://readthedocs.org/
• Split the documentation across various files
• Improve cmake documentation

[LecrisUT]

@pboettch Could you organize the remainder of the documentation so that I can handle the remainder of the cmake part?

[JacobCrabill]

You can use triple backticks here instead of ::: to play nicely with mdformat