feat(docs): add light/dark mode support for images and icons

[devin-ai-integration]

Description

Closes #8497

Adds support for light/dark mode variants for all image and icon fields in docs.yml configuration. Users can now specify different assets for light and dark themes using either a simple string path (backward compatible) or an object with separate light and dark properties.

Link to Devin run: https://app.devin.ai/sessions/8a0f273f51024c6e866e87ddb32e1601
Requested by: Catherine Deskur (catherine@buildwithfern.com)

Changes Made

• Added ImageConfig and IconConfig union types to the Fern definition that accept either a string or themed object
• Added ThemedImage and ThemedIcon interfaces to ParsedDocsConfiguration
• Updated configuration parsing with helper functions (resolveThemedIcon, resolveThemedImage, resolveIconConfigToSingleValue)
• Updated DocsDefinitionResolver with resolveIconFileId, resolveStringIconFileId, and resolveThemedImageFileId methods
• Updated ApiReferenceNodeConverter and toPageNode.ts to handle ThemedIcon type
• Updated file resolver to collect both light and dark image paths
• Regenerated TypeScript SDK from updated Fern definition
• Added CLI versions.yml entry for v3.31.0
• Updated README.md generator (if applicable)

Example usage:

products:
  - display-name: ProductA
    path: ./products/prod-a.yml
    image:
      light: ./assets/icon-light.svg
      dark: ./assets/icon-dark.svg

Updates Since Last Revision

• Fixed TypeScript compile errors in docs-resolver by updating resolveIconFileId to accept ThemedIcon | undefined
• Added resolveStringIconFileId for navbar links (FDR SDK types expect string | undefined)
• Added resolveThemedImageFileId for product images
• Updated ApiReferenceNodeConverter.ts and toPageNode.ts to handle ThemedIcon type
• Restored hidden property in PlaygroundSettings that was accidentally removed during SDK regeneration
• Reverted stream wrapper files (Node18UniversalStreamWrapper.ts, UndiciStreamWrapper.ts) that had unrelated formatting changes from SDK regeneration
• Fixed formatting issues flagged by biome

Testing

• Lint checks pass (pnpm run check)
• Local compile passes (pnpm -F @fern-api/docs-resolver compile)
• Unit tests added/updated
• Manual testing completed

Human Review Checklist

• Verify backward compatibility: simple string icons/images should still work (code duplicates value for both light/dark)
• Check if downstream consumers (docs renderer, FDR) need updates to handle the new ThemedIcon/ThemedImage types
• Known limitation: Navbar link icons use resolveStringIconFileId which extracts a single icon (prefers light, then dark) because FDR SDK types expect string | undefined. Full themed support for navbar links would require FDR SDK updates.
• Verify the stream wrapper file reverts don't affect functionality
• Confirm PlaygroundSettings.hidden property restoration is correct

[devin-ai-integration]

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