Shift to planning-specific singularity hub

Since singularity seems to be the leading virtualization technique for high-performance planning, and since the original hub has been discontinued, it's probably worth our while to have a community-led effort to host planners and planning-related software somewhere. Ala https://singularityhub.github.io/sregistry/

@FlorianPommerening : This something you guys would be interested in maintaining from Basel?