[DOC OCP] Refine the Policies and permissions in Keycloak Authorization Services documentation

Section 4.5.3, Managing policies and permissions in Keycloak Authorization Services is available in the Strimzi documentation onky.

This issue is to edit and modularise this content ready for publication with the AMQ Streams documentation.

There is a separate docs issue to refine the end-to-end example in section 4.5.4, "Example authorization rules configuration using Keycloak Authorization Services".