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
No idea. It's mentioned in two places: https://github.com/zerovm/zerovm/blob/f0220e2dc97bf0343e6f58ece08ae342ac5687a4/doc/channels.txt, https://github.com/zerovm/zerovm/blob/f0220e2dc97bf0343e6f58ece08ae342ac5687a4/src/channels/nservice.h.