support offline installation and per-site image configuration for Unbounded components

Publish end-to-end installation guidance for Unbounded CNI and other Unbounded components in offline or restricted-network environments. The documentation should explain how to mirror or preload required images and artifacts, configure registry authentication and image pull policies, and install the components without direct access to public registries such as GHCR or MCR.

Unbounded CNI and other Unbounded components should also support configuring image references per site. This would allow each site to use its own offline registry or registry endpoint, simplifying node network requirements and avoiding the need for every node to reach a single shared registry.