Add offline (air-gapped) installation guide for KubeStash

[tamalsaha]

Summary

Adds an Offline (Air-gapped) Installation section to the KubeStash Helm install docs (docs/setup/install/kubestash/helm.md).

The new guide covers:

• Mirroring the required images — using the installer repo's catalog helper scripts (copy-images.sh for connected mirroring, export-images.sh/import-images.sh for fully air-gapped transfers, and import-into-k3s.sh for registry-less k3s clusters).
• Verifying image paths are rewritten via helm template before installing.
• Installing from a private registry using global.registryFQDN (operator and helper images) and kubestash-catalog.proxies.ghcr (backup addon images).
• A pre-seed image list for registries that cannot mirror an entire namespace.

Test plan

• Verify rendered docs display correctly
• Confirm info.version param and links resolve

[github-actions]

Visit the preview URL for this PR (updated for commit 9b498dd):

https://kubestash-qa--pr87-offline-9b6wmzk5.web.app

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