Draft: docs: add note about using require-image-signature-policy [ci skip]

General MR

Adds a note to kyverno-policies values file about using the require-image-signature policy

Summary

If a user wants to use the require-image-signature policy, then their kyverno admission controller must have access to the public internet in some cases and their image registry. This MR adds a callout showing how to enable a network policy that allows kyverno admission controllers to reach out to the internet.

Considerations

I have also considered enabling this policy and the network policy by default.