docs(gpu-operator): clarify MPS support

[efegokdemir]

Summary

Closes #387.

Clarify the GPU Operator's MPS support boundaries so readers do not mistake the existing ClusterPolicy/device-plugin time-slicing guide for official MPS support.

Changes

• Document that ClusterPolicy/device-plugin GPU sharing is supported through time-slicing and MIG, while MPS is not officially supported through that integration.
• Link the MIG User Guide for application considerations on statically partitioned MIG instances.
• Document experimental MPS through the DRA driver's MPSSupport feature gate and link the DRA compatibility guidance.
• Remove the misleading MPS claim from the getting-started next steps and point readers to the DRA documentation instead.

Testing

• git diff --check — passed.
• Changed-line width check — passed.
• Sphinx documentation build was attempted with ./repo docs -p gpu-operator, but the repository bootstrap selected an x86_64 Python binary on this arm64 macOS host (Bad CPU type in executable).

Notes

The classic time-slicing and MIG documentation remains unchanged. No existing open PR touches the affected GPU Operator documentation files. AI assistance was used to investigate and prepare this contribution; the author reviewed the changes and takes responsibility for the submission.