Automate creation of the coordinated package list

[eteq]

Inspired by #74 - should there be some automated machinery to populate the coordinate package links list that is derived from the canonical source of coordinated packages? (i.e. the json file that creates https://www.astropy.org/affiliated/index.html which is at https://github.com/astropy/astropy.github.com/blob/main/affiliated/registry.json)

[Cadair]

Making this dynamic in js would be a good chunk of work. We could probably write some code which did it at sphinx build time, we'd need to decide what we wanted the fallback to be if the json couldn't get fetched?