fix: update existing preview comment instead of creating new one

[devin-ai-integration]

fix: upsert preview docs comment instead of creating duplicates

Summary

Adds comment_tag and mode: upsert to the thollander/actions-comment-pull-request step so that subsequent pushes to a PR update the existing preview comment rather than posting a new one each time.

Review & Testing Checklist for Human

• Verify that on a PR with multiple pushes, the preview comment is updated in place rather than duplicated
• Confirm comment_tag: preview-docs and mode: upsert are supported parameters in thollander/actions-comment-pull-request@v2.4.3

Notes

• The same fix was already applied to fern-api/docs (preview-docs.yml L149-150); this PR brings the docs-starter template in sync.
• A companion PR in fern-api/fern-platform updates the dashboard template and generated-templates.ts to match.
• Requested by: Catherine Deskur
• Link to Devin run

[devin-ai-integration]

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