Add buf installation to GitHub Actions workflows

[devin-ai-integration]

Summary

Adds the bufbuild/buf-setup-action@v1 step to all three GitHub Actions workflows (check, preview-docs, publish-docs) so that buf is available when Fern commands run. This is needed for gRPC/protobuf processing to work correctly, which should fix the preview and production docs generation.

Review & Testing Checklist for Human

• Verify CI passes on this PR (the workflows will run and test themselves)
• After merging, create a test PR to confirm the preview URL generates correctly
• Verify production docs publish successfully after a push to main

Notes

The buf CLI is required for Fern to process protobuf files in this gRPC demo repository. Without it, the docs generation fails.

Link to Devin run: https://app.devin.ai/sessions/14504afa04114b0ebff10ec7db1a916a
Requested by: Catherine Deskur (catherine@buildwithfern.com)

[devin-ai-integration]

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[github-actions]

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