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
Yes, the actual exit code of the process != EDQUOT; it's something else. I've only very briefly experimented with exceeding the channel limits, and only with Python. It's not super clear what the behavior is/should be.