nfs: update documentation and example (bp #6440)

[mergify]

This is an automated backport of pull request #6440 done by Mergify

Cherry-pick of c22189b has failed:

On branch mergify/bp/release-1.4/pr-6440
Your branch is up to date with 'origin/release-1.4'.

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Changes to be committed:

	modified:   Documentation/nfs.md
	modified:   cluster/examples/kubernetes/nfs/nfs-ceph.yaml
	new file:   cluster/examples/kubernetes/nfs/psp.yaml
	new file:   cluster/examples/kubernetes/nfs/rbac.yaml

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   cluster/examples/kubernetes/nfs/nfs.yaml

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[galexrt]

Closing in favor of #6455