Policy readme should have better TOC

As @dhomeier pointed out in https://github.com/astropy/astropy-project/pull/460#issuecomment-2648725084 , the README under policies folder is very team specific (at the time of writing this) but not all policies are controlled by the listed teams. For instance, a voting member can initiate new role staffing. Therefore, the README should be reorganized to include better TOC with all active policies.

Start with the README under the policies folder and review the active policies it references. Reorganize the README around a table of contents that includes all active policies, including policies not controlled by the listed teams; the work is done when readers can find every active policy from the README.

Written by the indexing model from the issue text.