Document how to verify image signatures in an air-gapped environment

[dervoeti]

Fixes https://github.com/stackabletech/issues/issues/437

I switched out Kyverno for the Sigstore Policy Controller, since the support for air-gapped environments is more mature than Kyverno's (at the moment at least) and added some guidelines and explanations on how signature verification in an air-gapped environment can be done.

Notes for the reviewer: The whole topic is not trivial and maybe not easy to reproduce, this documentation might be helpful as well: https://github.com/stackabletech/documentation-airgapped-setup It's a step-by-step guide on how to setup an air-gapped cluster on IONOS and how to configure the Policy Controller with a TUF mirror.

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