Port documentation to sphinx

[larsbutler]

I think it would great if we could port the existing documentation (in doc/), which is a very thorough and helpful, to a sphinx-based project. This would allow us to include the detail documentation on http://zerovm.readthedocs.org, in a similar fashion to zpm, etc.

What do you folks think? If everyone thinks this is a good idea, I can start working on it.

(Shamelessly copied from zerovm/zerovm#269.)