docs: document defaultOpen property for Accordion component

[devin-ai-integration]

docs: document defaultOpen property for Accordion component

Summary

Added documentation for the defaultOpen boolean property that was recently added to the Accordion component in fern-platform#4376. This property allows developers to specify which accordions should be expanded by default when the page loads.

Changes:

• Added defaultOpen to the Properties section in accordions.mdx with type, default value, and description
• Added live examples showing defaultOpen={true} in action in both accordions.mdx and accordion-groups.mdx
• Added a new "Default open state" accordion section in accordion-groups.mdx explaining the use case

Review & Testing Checklist for Human

• Critical: View the actual docs pages (accordions and accordion-groups) to verify the examples with defaultOpen={true} are actually open by default when the page loads
• Verify the property documentation accurately reflects the implementation (boolean type, defaults to false)
• Check that the code examples are syntactically correct and consistent across both files

Test Plan

1. Preview the docs site with these changes
2. Navigate to the Accordions and Accordion Groups pages
3. Verify that the "Accordion Open by Default" example and "Basic usage" accordion are expanded on page load
4. Toggle them closed/open to ensure they still work interactively

Notes

• This documents the feature implemented in feat: add defaultOpen property to Accordion component fern-platform#4376
• Link to Devin run: https://app.devin.ai/sessions/05d41add97944da4954d1323af845347
• Requested by: Deep Singhvi (@deep Singhvi)

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