KubeOne: No instructions for air-gapped setups

KubeOne supports air-gapped environments per se, but we don't have good documentation backing it up. We have [this section](https://docs.kubermatic.com/kubeone/v1.7/guides/registry-configuration/#preloading-images) which mentions the `preload-images.sh` script, but that page is deprecated in favour of [Using Mirror Registries](https://docs.kubermatic.com/kubeone/v1.7/guides/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.

Start with the deprecated Preloading Images section and the Using Mirror Registries guide, then inspect the preload-images.sh script and the systemPackages.configureRepositories option. Document how air-gapped mirror configuration and node package repositories work, including whether binaries can be baked into VM images and what users must configure for provisioning to succeed.

Written by the indexing model from the issue text.