fix: correct Install skills learn-more URL

[devin-ai-integration]

Summary

The "Learn more" link in the Install skills modal (page-actions.options.skills.learn-more-url in fern/docs.yml) pointed at /learn/docs/ai/agent-skills, which 404s — the Agent skills section lives under the ai-features section slug, and no redirect covers this path (fern/docs.yml has /learn/docs/ai/… redirects for the renamed writer/llms-txt/mcp-server pages, but not for agent-skills).

-learn-more-url: https://buildwithfern.com/learn/docs/ai/agent-skills
+learn-more-url: https://buildwithfern.com/learn/docs/ai-features/agent-skills

Link to Devin session: https://app.devin.ai/sessions/c6e2fe58cff44473ae2c990f1dfb399b

[devin-ai-integration]

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[github-actions]

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