Add airgap import docs for the hauler archive

Documents importing bb-<tag>-images-charts.tar.zst into a downstream registry for airgapped installs, and deploying Big Bang from it.

The deploy half is built around the distinction that images and charts reach the registry by different routes: images through a containerd registry mirror with their references left as registry1.dso.mil/..., charts through helmRepositories in values. registryCredentials does not rewrite image references — assuming it does sends every pod to ImagePullBackOff while the rendered manifests look correct.

Also covers the sourceType default of git, the values schema requiring credentials for an unauthenticated repo, the TLS and source-controller CA requirement, Harbor single-project rewriting, and why EKS/ECR needs reference rewriting rather than a mirror.

Includes a section on unpacking the archive without the hauler CLI, since it is a standard OCI image layout that skopeo can push directly.

Verified end to end on k3d against the 0.0.7 archive: the archive pushes into a TLS registry (189 repositories), its OCI charts pull as real Helm charts, all four Flux controllers start from it, and metrics-server reached Helm install succeeded with its image reference left untouched as registry1.dso.mil/....

Docs only.

Draft: the artifact is not published by a real release until big-bang/pipeline-templates/pipeline-templates!1535 (merged) lands.

Related to big-bang/pipeline-templates/pipeline-templates#1102 (closed)

Closes #3435 (closed)

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