docs: Missing examples of storage configuration in the manifest reference

The [manifest reference](https://canonical-openstack.readthedocs-hosted.com/en/latest/reference/manifest-file-reference/) lists 3 supported storage backends: Dell Storage Center, Hitachi, and Pure Storage.

According to the Cinder snap readme, there are other supported storage backends which should be added to the reference:

- [Dell PowerStore](https://github.com/canonical/snap-cinder-volume#dell-powerstore-backend)
- [HPE 3Par](https://github.com/canonical/snap-cinder-volume#hpe-3par-driver-backend)
- [Infinidat InfiniBox](https://github.com/canonical/snap-cinder-volume#infinidat-infinibox-backend)