Document package mirror requirements for air-gapped / offline mode

[embik]

Our documentation page "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.