Add note about contacting Fern to enable custom React components

[devin-ai-integration]

Summary

Adds a note to the custom React components documentation page informing users that they must contact Fern to enable this feature for their account. The note is placed prominently after the Pro plan snippet and before the main content.

Review & Testing Checklist for Human

• Verify the contact link (https://buildwithfern.com/contact) is the correct URL for users to reach out
• Confirm the wording accurately reflects the intended message about needing to contact Fern

Notes

• Session: https://app.devin.ai/sessions/367ad74dcac04f09a3732f9c3df7f769
• Requested by: Fern Support ([email protected])

[devin-ai-integration]

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