Update bigbang Airgap Zarf documentation

📌 Update Big Bang Airgap Zarf Documentation to Reflect Current Tooling and Practices

Recent investigation under Issue #152 confirmed that Zarf remains a viable method for delivering Big Bang into airgapped environments — but the current Airgap Zarf documentation is outdated, misleading, and insufficient for practical implementation.

We validated deployment using recent Zarf versions without relying on the deprecated Big Bang Zarf plugin. Instead, we successfully authored a working zarf.yaml based on modern Zarf examples and manual configuration.

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❌ Remove / Deprecate

• References to generate-big-bang-zarf-package (unmaintained, no longer needed).
• Any assumptions that upstream Zarf continues to support Big Bang natively.
• Legacy quickstart content, particularly:
  • airgap_quickstart.md — document is 3+ years old and obsolete.

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✅ Add / Update

Prerequisites

• Require Big Bang ≥ 3.1.
• Document necessity of building and maintaining custom Zarf packages.
• Clarify that Zarf plugin is deprecated; use core Zarf functionality.

Modern Workflow

1. Show how to simulate an airgapped environment by setting up two k3d clusters:

   • One build cluster with internet access.
   • One simulated airgap target cluster.

2. Use Zarf to init and bootstrap registry/git credentials:

   zarf init --components=git-server
   zarf tools get-creds
   

3. Build packages from the non-airgapped cluster using custom zarf.yaml.

   zarf package create . --confirm

4. Transfer packages to the airgapped cluster via scp or physical media (real or simulated).

5. Deploy to the airgapped cluster with:

   zarf package deploy zarf-package-*.tar.zst

Sample zarf.yaml

Include an up-to-date example containing:

• flux and bigbang components.
• Helm manifests, images from Iron Bank.
• HelmRelease-based healthChecks.
• zarf tools get-creds values included in bb-zarf-credentials.yaml.

Troubleshooting Tips

• Watch deployments using k9s or similar kubernetes monitoring tooling — Zarf may hang during health checks silently.
• Use zarf package inspect after build to validate content.

🧩 References

• Zarf Proposal: Remove Big Bang Plugin
• zarf-dev/zarf#2875
• zarf-dev/zarf#3059

📋 Deliverables

• Rewrite airgap-zarf doc for Big Bang 3.x with tested workflows.
• Remove or archive airgap_quickstart.md.
• Add complete, validated zarf.yaml example for users to build from.