docs: add Fern Writer capabilities section

[devin-ai-integration]

docs: add Fern Writer capabilities section

Summary

Adds a new Capabilities section to the Fern Writer docs page, placed between the existing "How it works" and "Setup" sections. The current page described basic usage and setup but lacked detail on what Fern Writer can actually do.

The new section covers six capabilities with dedicated subsections:

• Write and update content — new pages, sections, Fern component support, GitHub PR/issue context
• Reorganize and restructure — move/merge/split pages, navigation config updates, redirects
• Review and fix content — auditing for broken examples, outdated versions, inconsistencies
• Match your writing style — style analysis and AGENTS.md customization
• Iterate on feedback — Slack thread and PR comment workflows
• Attachments and context — image/file attachment support in Slack

No navigation or config changes — this is a content-only addition to an existing page.

Updates since last revision

• Fixed Vale Microsoft.Wordiness lint suggestion: changed "extract relevant details" → "understand the changes" in the "Write and update content" subsection.

Review & Testing Checklist for Human

• Content accuracy: Verify that each capability described accurately reflects what Fern Writer can do today. These were inferred from existing docs and the blog post, not from a feature spec — particularly the claims about updating docs.yml navigation config and adding redirects automatically.
• Redundancy check: The "Attachments and context" subsection overlaps almost verbatim with the sentence in the "How it works" section ("Fern Writer supports image and file attachments…"). Decide whether the duplication is acceptable or if one mention should be trimmed.
• Rendered output: Preview the page at the preview link to verify heading hierarchy, formatting, and that the new section flows naturally between "How it works" and the video accordion.

Notes

• Requested by: Fern Support
• Link to Devin run

[devin-ai-integration]

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[fern-api]

Requested by: kenny
Slack thread: View conversation

[github-actions]

📝 [vale] _{reported by reviewdog 🐶}
[Microsoft.Wordiness] Consider using 'remove' instead of 'extract'.

[github-actions]

🌿 Preview your docs: https://fern-preview-b3bf282f-f320-4740-8e1e-60e6540e2350.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/fern-writer