Onboarding for new maintainers and admins

We have different teams with different access priviledges. For each tier, rules and expectations should be made clear and onboarding be done for:

* New maintainers and admins.
* Existing ones: A refresher course, especially when rules/expectations have changed over time.

Example: One shall not open feature branch upstream, but always from fork. (And so on...)

Might be relevant:

* https://github.com/astropy/astropy-project/blob/main/policies/adding-new-role-members.md
* https://github.com/astropy/astropy-project/blob/main/policies/permissions.md
* https://github.com/astropy/astropy-project/blob/main/policies/pull-request-review-and-merge.md
* https://www.astropy.org/contribute.html
* https://docs.astropy.org/en/latest/index_dev.html

Open questions:

* How are people in the project currently onboarding their new peers?
* What is working well?
* What can be improved?
* If we generate new documentation and/or policies, where should they live?

Start by reading policies/adding-new-role-members.md, policies/permissions.md, and policies/pull-request-review-and-merge.md, along with the contributor documentation linked in the issue. Gather how new maintainers and admins currently onboard, what works, and what needs improvement. Done means an agreed onboarding and refresher approach is documented with a decided home for the new material.

Written by the indexing model from the issue text.