schemadoc: Rewrite to use migration store

Created by: efritz

This PR updates the schemadoc utility to use the migration store Describe method to print out the schema rather than relying on the \d+ functionality of psql (which changes version-to-version, hence requires us generate the schema in a fresh Docker container).

This PR will also unblock us having a more useful format for the schema.md files, which we can change arbitrarily without having to alter formatted strings directly.

Test plan

Manually checking the difference of internal/database/*schema.md files.