docs: document GPU Operator network prerequisites

[efegokdemir]

Summary

Documents the outbound network access required by the GPU Operator installation workflow.

This addresses #84 by making the two NGC endpoints and required HTTPS access explicit for clusters behind a firewall or proxy.

Changes

• Added nvcr.io as the container image registry endpoint.
• Added ngc.download.nvidia.com as the driver-download endpoint.
• Documented TCP port 443 and the client/worker scope of the requirement.

Testing

• git diff --check ✅
• reStructuredText content check for both endpoints and the HTTPS requirement ✅
• Full Sphinx documentation build was unavailable because sphinx-build is not installed.

Notes

The change is limited to the GPU Operator installation prerequisites and does not alter installation commands.

This PR was prepared with AI assistance and reviewed against the issue request, existing air-gapped documentation, and the repository contribution guide.