Document package mirror requirements for air-gapped / offline mode

Our documentation page ["Offline Mode"](https://docs.kubermatic.com/kubermatic/v2.23/installation/offline-mode/) which documents steps to set up an air-gapped KKP installation is missing some crucial details.

One of them is a list of OS package repositories (deb/rpm) that need to have a local mirror within the air-gapped environment available. This should - from a quick check - be mostly the base OS repositories and the docker.com repository for containerd, but this should be confirmed.

## Acceptance Criteria

- "Offline Mode" documentation lists all OS package repositories needed and a rough explanation on how to configure them as mirrors (without tying it to a specific product).
- "Offline Mode" documentation references the need to create a custom OperatingSystemProfile and how to reconfigure it to use the local package mirrors.

Start with the linked “Offline Mode” documentation page and verify which base OS and docker.com package repositories are required for an air-gapped KKP installation. Document all required deb/rpm repositories and generic mirroring guidance, then explain how to create and reconfigure a custom OperatingSystemProfile to use the local mirrors; the acceptance criteria define done.

Written by the indexing model from the issue text.