Enable multi-source docs for cuVS Fern instance

[aschilling-nv]

Summary

• Adds multi-source: true to the Fern docs instance config in fern/docs.yml
• Enables cuVS to publish independently to its sub-path (docs.nvidia.com/cuvs) within a shared NVIDIA domain
• Consistent with the Fern multi-source docs pattern; the instance already has the required global-theme and basepath-aware config in place

Test plan

• Verify Fern docs build succeeds with multi-source: true
• Confirm cuVS sub-path publishes independently without affecting other NVIDIA sub-paths

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