KubeOne: No instructions for air-gapped setups

[embik]

KubeOne supports air-gapped environments per se, but we don't have good documentation backing it up. We have this section which mentions the preload-images.sh script, but that page is deprecated in favour of Using Mirror Registries which is utterly unhelpful with air-gapped setups. It doesn't even explain how I could point all mirrors to my mirror (if that is even possible).

In addition, we have systemPackages.configureRepositories but it's nowhere documented. How do you use it? How can you make sure that if you don't configure repositories, your nodes get provisioned? Can you bake your binaries into the VM image?

This is kind of important because air-gapped is a high-profile use case for KubeOne and KKP.