docs: add Files UI component documentation

[devin-ai-integration]

docs: add Files UI component documentation

Summary

Added comprehensive documentation for the new Files UI component (introduced in fern-platform PRs #4599 and #4926). The documentation covers the <Files>, <Folder>, and <File> components used to display interactive file tree structures in documentation.

Changes:

• Created files.mdx with usage examples, three variants (nested folders, linked files/folders, default open), and complete property reference
• Added Files component to navigation in docs.yml (placed alphabetically between "Endpoint schema snippet" and "Frame")

Review & Testing Checklist for Human

• CRITICAL: Verify components render in preview deployment - The Files components showed errors when testing locally because the local dev server doesn't have the latest fern-platform code. Check the preview deployment to ensure all four examples render correctly with proper styling, icons, and interactions
• Test interactive functionality - Verify folders expand/collapse on click, chevron rotates correctly, and defaultOpen property works as expected
• Test linked examples - Verify the href property works on both files and folders with correct underline-on-hover behavior
• Verify navigation icon - Check that fa-duotone fa-folder-tree displays correctly in the sidebar
• Cross-check properties - Compare documented properties with actual component implementations in fern-platform PR #4599 and PR #4926 to ensure accuracy

Test Plan

1. Open the preview deployment and navigate to the Files component page
2. Interact with each example to verify folder expansion, links, and visual styling
3. Compare rendered output with reference screenshots from the original fern-platform PRs

Notes

• Documentation structure follows existing component docs (accordions.mdx, steps.mdx)
• Local testing showed "Something went wrong!" errors because the Files components aren't available in the local dev environment - this is expected and should work in the preview/production deployment
• This documentation depends on fern-platform PRs #4599 and #4926 being deployed to production

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Link to Devin run: https://app.devin.ai/sessions/c55d625c9a6544f197f9351f8cd2ee09
Requested by: Catherine Deskur ([email protected])

[devin-ai-integration]

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