Document data model as JSON schema

[swissspidy]

This is basically #11323 but for JSON schema

Documenting our data model this way would make maintenance easier and increase DX due to the added tooling support.

For example, we could make all our templates and text sets reference this schema.

It's also possible to automatically create TypeScript typings from JSON schema, which might be useful as well. Though we probably want to create those manually for now since everything is split up across multiple packages.