litewitness: Document database schema

This would be helpful for operators that don't fancy reading Go code. Doesn't have to be long, but something would go a long way.

Start by locating the litewitness database schema in the Go code and identify the operators-facing schema details that need explanation. Document the schema clearly enough that operators do not need to read the Go implementation; the issue does not name a specific file or test to run.

Written by the indexing model from the issue text.