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
Maybe you want to write "ZeroVM will exit with a status of EDQUOT (quota excdeeded)" since programs (which `zerovm` is) exit, they don't really raise errors.

When reading this, I'm also wondering what exit status EDQUOT is. I looked in [sysexists.h](http://www.freebsd.org/cgi/man.cgi?query=sysexits) (the only standard list of exit codes I know of) but didn't find it defined there.