Geo: Add developer guidance for adding Blob types

I assume most devs will come across https://docs.gitlab.com/ee/development/file_storage.html when adding new Blob types.

Proposal

• Add a section "Adding a new type of file?"
  • Don't reuse uploads table anymore because...
  • Immutable paths are better because...
  • file_store column should be non-nullable because...
  • Object storage support is required because... (so reuse existing code and don't roll your own)
  • Geo support is required because... (and link to framework doc)
  • Maybe link to an example MR?