docs: Optimize repository for publishing to Cue registy

[eddie-knight]

This was intended to address #154

I went pretty far down this path today before trying to sign in to the cue registry. I entirely forgot that I tried at some point in the past and it became an obstacle:

This PR:

• Move schema.cue to the root level, so that the CUE module can be "github.com/ossf/security-insights"
• Moved the other contents of schema/ to docs/
• Adds anti-examples for testing, and a make test command to ensure validation behaves as expected for good and bad
• Adds docs for users to take advantage of the module (WIP, generated)
• Adds maintenance docs for publishing to the CUE registry (WIP, generated)