Update affiliated package precedures to state how potential reviewer list is updated

[pllim]

From Astropy Coordination Meeting 2023: Reviewer list (which is private by design) is hard to update unless you are already deep in the astronomy field and know who is who. @hamogu has updated developer survey to add a question for people to sign up (or remove themselves) as reviewers, to ease burden off Editors.

Unsolved: What about people who can review but do not get our dev survey? @hamogu offered to help Editors directly to do this in private but is this sufficient?

[pllim]

Additional idea: What if we let the people getting the survey to volunteer other people they know who are not necessarily in Astropy?

[hamogu]

Current rules say “The pool of available reviewers will be anyone who has an official role on the Astropy team or anyone who has participated in the Astropy project enough to be familiar with the project guidelines and requirements” (https://github.com/astropy/astropy-project/blob/main/affiliated/affiliated_package_review_process.md) - which is in general the people who get the roles survey or got it in the past.

That does not mean that we can't change that - but it's worked in past so we need a reasons to change it.

[pllim]

Personal opinion as ex-editor: I am hesitant to pull people not familiar with the Project already, because I feel like you at least need to be involved, even a little, to be an effective reviewer. So I agree with @hamogu .

[pllim]

Might be superseded by #334 but not sure yet.