Document how sources are mirrored to the Data Sources App from the Source Manager

[josh-chamberlain]

Context

• Part of Make source manager the "source of truth" for the database #868

We're syncing over and displaying just some of the many URLs in the source manager. We should explain how we are choosing which sources to sync over.

Requirements

• document (in the API docs? somewhere else?) what the Great Filter looks like which decides what is synced, and what criteria a URL must meet to appear in the db
  • can point directly to the relevant code
  • should also human-words-describe it

[josh-chamberlain]

Fixed by Police-Data-Accessibility-Project/data-source-manager#537