Add pull request attribution section to Fern Writer page

[devin-ai-integration]

Summary

Adds a new "Pull request attribution" section to the Fern Writer documentation page (fern/products/docs/pages/ai/writer.mdx). The section explains:

• That each PR created by Fern Writer includes a Requested by field in the description for traceability
• That commits are signed and attributed to fern-support to distinguish automated changes

The section is placed between the "How it works" content and the "Setup" section.

Review & Testing checklist for human

• Verify the description of the "Requested by" field accurately reflects Fern Writer's actual behavior (e.g., does it always attribute to the requesting person/team, or does it default to "Fern Support"?)
• Confirm the placement between the video accordion and Setup section reads well in the page flow
• Preview the page to check rendering of the bold formatting and inline code

Notes

• Vale lint passes with no new warnings (the only warning is pre-existing on line 26 re: "soon")
• Devin Session
• Requested by: Fern Support

[devin-ai-integration]

🤖 Devin AI Engineer

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✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
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[fern-api]

Requested by: kenny
Slack thread: View conversation

[github-actions]

🌿 Preview your docs: https://fern-preview-452f275a-07f4-4a9d-b39a-0b1c897a351d.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/fern-writer