policy api doc for review only

@croche-okta @wdawson-okta @srinivasanagandla-okta @stsai-okta @lboyette-okta @hoanguyen-okta @karlmcguinness-okta @cwu-okta @tthompson-okta

This is the doc review for the policy api. This is a new doc. Please look at it carefully, because I think a lot of things might need adjustments. Since this isn't in postman yet, I didn't have the objects for the reponses. If it's too much text to add, we can meet about it.
Not entirely true... `system` means that the policy or rule was created by the system and not by the user. For now, the only policy which has this attribute is the `default` policy. I think we are ok leaving it how it is for now. But I'd try to avoid language implying that if a policy has a `system` attribute then it is THE default policy as in the future there could be other policies with `system`