docs: add air-gapped environment guidance for private registry

[Shivampal157]

This PR adds documentation for deploying container images in air-gapped environments.

It explains the requirements for using a private container registry and ensures
Knative Serving workloads can pull images without external internet access.

Happy to make changes if needed.

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The committers listed above are authorized under a signed CLA.

• ✅ login: Shivampal157 / name: Shivam Pal (26e8e4e)

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Shivampal157
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[knative-prow]

Welcome @Shivampal157! It looks like this is your first PR to knative/docs 🎉

[Shivampal157]

Thanks for the review!
I’ve signed the Individual CLA. Please let me know if any changes are needed.