Created by: pixeldesu
This PR picks up the "Creating custom registries" bullet point from #2256
While writing this, I found that in the undertaker README, registry() is partially explained already, but I felt that just copy-pasting wasn't enough and the document here is a bit more thorough on what's exactly required or happening.
I also decided to link to all of the PoC registries that have been created in the organization beforehand, so people have some more starting points.