Switch to using a single quay.io organization for all non-GCP imagebuilding hubs

[yuvipanda]

Our current documentation for enabling imagebuilding feature asks us to use a registry for each project on GCP (which makes sense), but also create a new org on quay for each new hub. This adds more complexity but doesn't actually buy us anything, as there are no private repos being built here. Plus, it means that when two hubs build the same image, they have to rebuild it unnecessarily.

Instead, let's switch to using one quay.io org for all imagebuilding hubs we maintain on non-GCP. If the hub wants to leave us, then we can use right to replicate and make a new org.

Definition of done

• New quay.io org used for all non-GCP hubs is created
• All existing non-GCP imagebuilding hubs point to this new quay.io org
• Documentation for enabling imagebuilding changed