Add "How Fern uses AI" cross-product page

[devin-ai-integration]

Summary

Adds a single customer-facing page that inventories every AI capability across Fern's products, for customers (and Fern's legal team, working through the updated Terms of Service) who need one place that answers: what AI runs, what business problem it solves, what Fern stores, and how to turn it off.

New page: fern/products/home/pages/ai.mdx → published at /learn/home/ai, linked from the Home landing page CTA row and from the Docs AI overview intro. Changelog entry added under fern/products/docs/pages/changelog/.

Content is grounded in the platform/CLI source rather than in existing docs prose, and deliberately separates three categories that were previously blurred together:

• capabilities that call a model (Ask Fern and its Slack app, MCP server, search suggestions, AI-generated examples, Fern Writer, Ask Fern analytics, four dashboard capabilities, fern sdk-diff),
• features that are fully deterministic and involve no model call (llms.txt, Markdown access, agent directives, API catalog discovery, agent skills),
• ordinary SDK/docs generation, which is deterministic and is stated as such.

The page names no providers or model versions, by request: the audience needs capabilities and business use cases, and a provider/model table would go stale every time a provider ships a version. What remains for the legal review is the "Data Fern sends and stores" section — what data leaves Fern per capability, query persistence and the configurable 1–3650 day retention window, the no-training statement — plus a table of the control that disables each feature.

Setup instructions are not duplicated: every capability row links to its existing canonical page.

Note on the URL: the page lives in the Home product, which resolves at /learn/home/... in the generated site. AGENTS.md states Home pages resolve at /learn/{page-slug}; that isn't what the CLI produces. Making /learn/ai work required a product-level slug override that broke /learn itself (redirect to /learn/home → 404), so this PR links /learn/home/ai and leaves routing alone. Worth fixing separately if the short URL matters.

Rendered at /learn/home/ai in the preview build; all internal links verified against a running preview; pre-commit (Vale) clean and fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/bad46565be904b3b8be9c63da0241d38
Open in Devin Desktop: https://app.devin.ai/desktop/session/bad46565be904b3b8be9c63da0241d38?variant=devin

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1787598000-how-fern-uses-ai.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/overview
• learn/docs/changelog/2026/8/24
• learn/home/ai
• learn/home