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
Interesting. Both sources talk about error numbers, but I thought we were talking about a process exit status here?

I'm distinguishing because error numbers are something internal to a process and they're not normally mapped directly to the process exit code. As an example: `grep foo no-such-file` and `grep foo unreadable-file` both give an exit code of 2, but doing the same with `cat` gives an exit code of 1. The error numbers are `EPERM = 1` and `ENOENT = 2` in these cases.

Maybe @pkit or @bortoq can elaborate here?