Document how to use PodSecurityAdmission on Ingress-NGINX Chart

[rikatz]

As we are dropping PSP on #10608 we need to document how to use PSA on Ingress-NGINX.

We are already doing the tests using the "baseline" PSA, and basically we should state for users that they should label the namespace properly.

Because helm does not support today labelling the created namespaces, the documentation on Helm should be explicit on this need (as soon as Helm supports it, we can use the new approach).

[rikatz]

/help
/good-first-issue
/triage accepted
/priority important-long-term
/kind docs

[k8s-ci-robot]

@rikatz:
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[k8s-ci-robot]

@rikatz: The label(s) priority/important-long-term, kind/docs cannot be applied, because the repository doesn't have them.

In response to this:

/help
/good-first-issue
/triage accepted
/priority important-long-term
/kind docs

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.