Policy readme should have better TOC

[pllim]

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.