Document how to manage PVCs in RHDH Operator

Objective includes:

• To be able to manage (create/delete) by Operator PVC(s) to use by Backstage instance as a part of default configuration (e g will come as part of RHDH profile for particular Operator version)
• To be able to mount PVCs to backstage container easily, the same as we do for CMs, Secrets.

User Scenarios

• Using non-ephemeral PVCs for dynamic-plugins https://issues.redhat.com/browse/RHIDP-3572
• Mount PVC driven volume for Audit logging
  https://issues.redhat.com/browse/RHIDP-3241
• Mount additional assets (images for ex) for RHDH instance
  https://issues.redhat.com/browse/RHDHBUGS-58
  See also: https://docs.google.com/document/d/1u5JW4oMiLXwWgswh_GWejinThd0EXhHc_lUqx0tFP-4/edit?usp=sharing

References

• Upstream docs on mounting PVCs in the Custom Resource: https://github.com/redhat-developer/rhdh-operator/blob/main/docs/configuration.md#persistentvolumeclaims