docs: add file imports, assets, and API calls guidance for custom React components

[dannysheridan]

Summary

Adds documentation to the custom React components page addressing common user questions about file organization and capabilities. This was prompted by a user question in Slack asking about accessing other files, creating separate JS/CSS files, adding SVG assets, and making API calls with API keys.

New sections added:

• Importing files and assets - CSS imports, JS/TS module imports, SVG and image asset usage
• Making API calls - Client-side fetch example with security warning about API keys

Also adds a changelog entry for 2025-11-25.

Review & Testing Checklist for Human

• Verify technical accuracy of import patterns - The examples show standard React import patterns but should be verified against Fern's actual MDX component bundling behavior (CSS imports, SVG imports, module resolution)
• Validate mdx-components configuration guidance - The Note suggests adding utility and asset directories to mdx-components in docs.yml - confirm this is the correct approach
• Review API key security guidance - The suggestion to "pass keys as props from your MDX file, where you can use environment variables" may be misleading if MDX doesn't have access to server-side env vars

Recommended test plan: Create a test Fern project with a custom React component that imports a CSS file, a utility module, and an SVG asset to verify the documented patterns work as described.

Local Testing

Documentation renders correctly in local dev server:

View original video (rec-638680df7a8d45c79e4032ac2c5f9ca9-edited.mp4)

Notes

• Triggered by user question: https://buildwithfern.slack.com/archives/C094NCEAZ0V/p1764084856886689
• Link to Devin run: https://app.devin.ai/sessions/adaee77daef549f2be163ad89a65c20b
• Requested by: [email protected] (@dannysheridan)

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