Document how update works with airgap installations and related options to disable auto update

Task content

We are lacking a documentation for how airgap installation works, what it provides, what are limitations and known issues and how to configure it.

I would suggest to add a docs page (e.g. under Installation or Configuration) covering:

1. What an air-gapped/all-in-one build
2. How automatic updates behaves, and how to use preferences.update.appUpdate (or managed configuration) to control that behavior.
3. A pointer to managed configuration for admins who want to enforce this org-wide (preferences.update.appUpdate via locked.json/ default-settings.json).
4. The consequence of updating in-app from an air-gapped install to the standard variant (loss of the bundled machine base image), and how to avoid or plan for it. This Might be individual issue, feel free to open new one.

Related: #18189, https://github.com/podman-desktop/podman-desktop/issues/15384