Add general maintainers role for astropy core package

[hamogu]

This has been discussed in the dev telecon 2022-02-03 and there was general agreement to add such a role. This PR is a draft of that role description; unfortunately I don't remember exactly how we described this role in the telecon, so this is based on my notes, my recollection and the discussion in astropy/astropy-project#136

@bsipocz You had a very good description of this role in the last dev telecon, but I was not fast enough to write that down. Could you please have a look at this and comment or suggest changes to include what you said?

x-ref: astropy/astropy-project#136

[mwcraig]

LGTM

[eteq]

Happy with this broadly, but I've suggested what I can remember as part of the discussion in the telecon of the boundary between the generalist and the devops role.

[hamogu]

I've added "triaging" to this new role and, for consistency, also to the existing sub-package maintainers roles. I think it's unfair to leave that all to DevOps and there might be cases that that maintainers can actually triage better. a\Adding labels is also non-destructive, so if anyone adds a wrong one, somebody else can fix it any time. The problem is that we don't have enough people helping out (when @pllim is on vacation it's just not happening), not that too many people contribute.