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
• Bumps Fern version from 5.44.3 to 5.57.0 in fern/fern.config.json

Test plan

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

🤖 Generated with Claude Code

[cjnolet]

/merge