Add AI data and controls to the AI features and dashboard overviews

[devin-ai-integration]

Summary

Reworked from the original standalone "How Fern uses AI" page (now removed, along with its Home nav entry, landing-page CTA, and changelog entry) into additions to existing pages.

• fern/products/docs/pages/ai/overview.mdx: new Data and controls section with the cross-cutting no-training statement, links to each feature's data handling section, and a table of how to turn each feature off (dashboard Disable, page-actions.options.mcp, ai-examples.enabled, robots.txt). One sentence added to Optimize for AI stating those features are deterministic with no model call.
• fern/products/docs/pages/ask-fern/what-is-ask-fern.mdx: new Privacy and data handling section (what each request sends, RBAC-filtered retrieval, query storage and retention) mirroring the one on the Fern Agent page.
• fern/products/docs/pages/ai/ai-examples.mdx: one sentence on what generation sends to the model.
• fern/products/dashboard/pages/overview.mdx: new AI features section (cards + llms-only list) for site import, brand extraction, pull request summaries, and Fern Agent, with a one-line data statement.
• fern/products/docs/pages/changelog/2026-09-08.mdx: changelog entry.

Dropped from the original page: the capability inventory tables and "business use case" column (the overview's existing card sections already cover this), the CLI section (fern sdk-diff, fern mcp install), and the stale Fern Writer references.

Vale clean on changed lines (pre-commit), fern check --warnings reports 0 errors.

Link to Devin session: https://app.devin.ai/sessions/0a4de152127a4b9386de2f9866422282
Open in Devin Desktop: https://app.devin.ai/desktop/session/0a4de152127a4b9386de2f9866422282?variant=devin

[devin-ai-integration]

🤖 Devin AI Engineer

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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/dashboard/getting-started/overview
• learn/docs/ai-features/ai-examples
• learn/docs/ai-features/overview
• learn/docs/ai-features/ask-fern/overview