Document responsibilities of Nixpkgs Maintainers pertinent to issues

Hi there! I've come to notice that the Nixpkgs Maintainers team has access to managing issues in the nixpkgs repository, that includes:

1. Labeling issues
2. Closing issues
3. Re-opening issues

However there is no documentation of such grant nor the scope of the permissions. I find it important that this is written down somewhere so we know what Nixpkgs Maintainers are supposed to be doing to support nixpkgs as well to understand how we can contribute to the repository outside of package maintenance.

I've spoken to @infinisil who described to me that this permission was granted probably to manage issues created regarding packages you maintain. If so, perhaps introducing a bot similar to nixpkgs-merge-bot would be more beneficial than giving everyone access to managing issues.
I for one think that having at least a selection of maintainers outside of the committer team would help in the long run dealing with the flood of issues the repository gets.

Cheers! o/

No file, test, or entry point is named. Review the existing Nixpkgs and organisational documentation and the current issue-management permissions, then document the maintainers' responsibilities and permission scope. Done means contributors can find an agreed description of what Nixpkgs Maintainers may do and how that supports issue management.