DOC: Better documentation of roles and rights

The current documentation of the rights in 3scale is very limited

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.14/html-single/admin_portal_guide/index#give_new_users_rights

This raises a lot of questions for our customers when it comes to mapping their internal concepts to what can be achieved in 3scale atm. (I know there are plans to change roles/right concept long term)

It would be good to have something similar to what we see in OpenShift which shows the currently available rights/roles and their meaning in a nice table

https://access.redhat.com/documentation/en-us/openshift_container_platform/4.14/html/authentication_and_authorization/using-rbac#default-roles_using-rbac

In 3scale we seem to be mixing the terms rights and roles. From what I see we do have two roles (members/admins) and those have rights (admin = all rights, members = rights can be defined individually).