[Docs] Document compatibility matrix for CSI support

We need to document compatibility matrix for CSI support by sseago's comment on https://issues.redhat.com/browse/OADP-585:

My understanding talking to rhn-engineering-dymurray about this last week was that 1.0.3 will be supported on 4.11 for non-CSI use cases only, and 1.1 will be supported on 4.6 for non-CSI use cases only. As far as I know, for 4.7->4.10, both v1 and v1beta1 of the storage.k8s.io API are available, so either OADP version is consistent with CSI usage there. If you're not using CSI, then either OADP version should work on all supported version of OpenShift.

This restriction needs to be documented. Velero CSI plugin (and velero server) don't currently handle APIVersion negotiation for storage.k8s.io – Velero 1.8 and older (OADP 1.0.z) supports storage.k8s.io/v1beta1 only, and Velero 1.9 and later (OADP 1.1+) supports storage.k8s.io/v1 only. I suspect that this means we should recommend that customers that require CSI support remain on OADP 1.0 until they upgrade to a newer OpenShift version.