Docs overhaul

Docs now include:

- A more thorough introduction
- Core ZeroVM docs, with pretty decent detail (but more work is needed)
- A more thorough ZeroCloud overview
Here's one reference to it: https://github.com/torvalds/linux/blob/9a3c4145af32125c5ee39c0272662b47307a8323/include/uapi/asm-generic/errno.h. Another is the Python [`errno` module](https://docs.python.org/2/library/errno.html).