[BUG] Support for single server, single drive K8s setups

[djwfyi]

Describe the bug
Operator PR 1169 added support for single node, single drive deployments for testing purposes.

Update reference/operator-crd.html to indicate that this is allowed for testing purposes.
For supported production deployments, a minimum of 4 nodes is still required.

[ravindk89]

https://github.com/minio/operator/blob/master/pkg/apis/minio.min.io/v2/types.go#L621-L627

We ingest the CRD docs from the link above. Any changes have to be made here, which will then trickle down to our own docs.