docs: clarify NVIDIA AI Enterprise vGPU images

[efegokdemir]

Summary

Clarify that the source-build procedure in the vGPU installation guide is intended for users who are not NVIDIA AI Enterprise customers. Link NVIDIA AI Enterprise customers to the workflow for pulling the prebuilt vGPU driver image from NGC.

Changes

• Add the audience distinction before the driver-container build steps.
• Link the NVIDIA AI Enterprise installation guide for the prebuilt-image workflow.

Testing

• git diff --check — passed.
• Sphinx executable check — not run because sphinx-build is not installed.
• ./repo docs -p gpu-operator — attempted; blocked on this Apple Silicon host because the repository bootstrap selected an x86_64 Packman Python binary (Bad CPU type in executable).

Notes

• Rechecked issue install-gpu-operator-vgpu: say the driver-container build is for non-NVIDIA AI Enterprise customers; NVAIE customers can pull the prebuilt image from NGC #492 and the open PR list immediately before submission; no competing PR for this issue or page was found.
• The change is documentation-only and does not alter installation commands.
• AI assistance was used for repository research and drafting. I reviewed the complete diff and take responsibility for the submitted change.