Update Documentation for oc-mirror plugin on P.

Existing docs: 

https://docs.openshift.com/container-platform/4.13/installing/disconnected_install/installing-mirroring-disconnected.html

https://docs.openshift.com/container-platform/4.13/installing/disconnected_install/installing-mirroring-installation-images.html#prerequisites_installing-mirroring-installation-images

https://docs.openshift.com/container-platform/4.13/installing/disconnected_install/installing-mirroring-creating-registry.html#installing-mirroring-creating-registry

These docs point to "mirror registry for Red Hat Openshift" or registries like Quay for the disconnected mirror, which aren't available on P/Z. Might be good to document what options do exist for these arches.