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
I actually thought the images were unioned, i.e., that they were overlaid on top of each other. The job description `zpm` generates mounts two images at `/` as far as I know, and the full virtual filesystem that is created that way is a union of the filesystems present in the two images.