docs: restore #custom-files anchor on llms-txt overview page

[fern-support]

Summary

When #4966 broke the single llms-txt.mdx page into multiple pages, the "Custom files" section moved to /learn/docs/ai-features/customize-llm-output#serve-custom-files. This left /learn/docs/ai-features/llms-txt#custom-files (which customers and any external references relied on) without a target.

Restores a short "Custom files" section on the llms-txt overview page. The H2 regenerates the #custom-files anchor so existing links resolve, and the section points readers to customize-llm-output for the full reference to avoid duplicating content.

Triggered by an external report that a bookmarked link to the old anchor was no longer landing on the relevant content.

Review & Testing Checklist for Human

• Preview /learn/docs/ai-features/llms-txt#custom-files and confirm the page scrolls to the new "Custom files" section.
• Confirm the link from the new section to /learn/docs/ai-features/customize-llm-output#serve-custom-files resolves.

Notes

Only fern/products/docs/pages/ai/llms-txt/overview.mdx changes. No nav/config change; the new H2 reuses the existing overview page.

Link to Devin session: https://app.devin.ai/sessions/62f0a408a7384ad1b2635f91e1a2343c
Requested by: @cdonel707

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1776960096-restore-custom-files-anchor.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/ai-features/llms-txt

[devin-ai-integration]

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