Document add_staff_user_ids... config

[consideRatio]

Captured from interaction by @sgibson91 and @GeorgianaElena on slack:

I think we could add a section about what the two configs add_staff_user_ids_to_admin_user and add_staff_user_ids_of_type do to the main authentication docs page https://infrastructure.2i2c.org/hub-deployment-guide/configure-auth/. I believe these two are only documented by the schema currently?

I think it would be good if we have add_staff_user_ids_to_admin_user and add_staff_user_ids_of_type part of our auth config examples for GitHub and CILogon.

Action points

• Document add_staff_user_ids_to_admin_user and add_staff_user_ids_of_type in a section not specific to an authenticatior class
  • As part of this docs, clarify that the principle of all OAuthenticator derived classes is that users must be authorized one way, but doesn't have to be authorized in multiple different ways. To be listed in admin_users, part of allowed_organizations, allowed_domains, etc are all ways to be authorized (but not allowed_idps, see [CILogon] Rename allowed_idps to idps in a non-breaking way? jupyterhub/oauthenticator#683)
• Include example of the add_staff_user_ids_... config in the specific authenticator class examples.
  Resolved by final commit in Grant access for 2i2c members only via admin_users #3233
• Make sure CILogonOAuthenticator is documented to always include either Google or GitHub as an secondary identity provider as well
  Resolved by final commit in Grant access for 2i2c members only via admin_users #3233