Add buf installation requirement for gRPC local generation

[fern-api]

Summary

Documents that buf must be installed when using local-generation: true for gRPC proto files. The requirement is now documented in three locations:

1. grpc-specs.mdx snippet - Added note to the local-generation parameter description
2. automation.mdx - Added note in the GitHub Actions section about installing buf in CI workflows
3. overview.mdx - Added note in the "Set up your fern folder" section for local development

Review & Testing Checklist for Human

• Verify the buf installation link resolves correctly
• Verify the bufbuild/buf-setup-action link resolves correctly
• Check preview to confirm notes render properly in all three locations

Test plan: Visit the preview links for each page and confirm the notes appear correctly and links work.

Notes

Link to Devin run: https://app.devin.ai/sessions/6c65525e74b34108922a946f9cf69d84
Requested by: Fern Support ([email protected])

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