feat: add changed markdown pages to preview PR comment

[devin-ai-integration]

Summary

Enhances the preview docs workflow to detect which MDX pages changed in a PR and include direct links to those pages in the preview comment. This mirrors the same feature already present in fern-api/docs's preview workflow.

What changed:

• Added fetch-depth: 0 to checkout for full git history (needed for git diff)
• New step queries /api/fern-docs/get-slug-for-file on the preview URL to resolve changed .mdx files to their page slugs
• PR comment now lists links to each changed page alongside the preview URL
• All slug resolution failures are handled gracefully — falls back to just showing the preview URL

Review & Testing Checklist for Human

• Verify origin/main is the correct diff target — this workflow uses on: pull_request without branch restrictions, so PRs against non-main branches will still diff against origin/main. Confirm this is acceptable for the starter template.
• Test by opening a PR that modifies an .mdx file and confirm the comment includes page links (this cannot be verified without a real PR run)
• Confirm the /api/fern-docs/get-slug-for-file endpoint is available on preview URLs generated by the starter template's Fern config

Notes

• Requested by: kenny@buildwithfern.com
• Devin session
• A companion PR for the fern-platform docs-starter template is also being created

[devin-ai-integration]

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[github-actions]

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