Clear set scopes for Astropy products

[pllim]

Two ideas were floated around:

• Chart to group them into "actively being developed", "bug fix only", and so on.
• A dashboard like Astropy Project Dashboard that displays similar info as above, but it scraps the info from some special repo metadata (repo topics or a special JSON/YAML file at root dir).

Once set, the scopes should be communicated clearly to the rest of the Astropy community.

Also related: astropy/package-template#497