docs: expand GPU Operator security guidance

[efegokdemir]

Summary

Complete the remaining documentation work in #155 by incorporating the security guidance from closed PR #136 into the current GPU Operator Security Considerations page. The page now explains how administrators can prevent unprivileged workloads from selecting GPUs through NVIDIA_VISIBLE_DEVICES and clarifies the related security assumptions.

Changes

• Add configuration guidance for restricting the NVIDIA Container Toolkit and device plugin to device-plugin-provided GPU lists.
• Document the host-volume and privileged-container assumptions that remain important after enabling the restriction.
• Correct the duplicated wording in the NVIDIA security bulletin link and normalize the GPU Operator security-policy link.

Testing

• git diff --check — passed.
• HTTP checks for the two referenced security links — passed with HTTP 200.
• ./repo docs -p gpu-operator — blocked by the repository bootstrap tool on this Apple Silicon host: its bundled python 3.10.5-1-macos-x86_64 executable fails with Bad CPU type in executable.

Notes

• I checked issue Improve security docs #155, closed PRs Security considerations #136 and Add security page #161, and the open PR list; no competing open PR for this remaining checklist item was found.
• AI assistance was used during development; I reviewed every changed line and take responsibility for the submitted documentation.