Official policy for Astropy data server

Would be nice to have an official policy to refer to when people ask what can be uploaded to Astropy data server and the max file size limit allowed, what kind of data is in scope or not, and so on.

Maybe policy can also suggest some turnaround time for PR reviews for PRs that attempt to modify or add data. We have several PRs trying to modify `sites.json` that have gone stale.

Start by reviewing the existing Astropy project policy documents and the stale PRs that modify sites.json. Define a policy covering permitted uploads, maximum file size, data scope, and expected review turnaround. Done means the policy is documented clearly enough to answer these questions and guide future data-related PRs.

Written by the indexing model from the issue text.