docs: add beta banner frontmatter property documentation

[devin-ai-integration]

Update banner frontmatter property documentation

Summary

This PR updates the frontmatter documentation to replace the beta banner documentation with comprehensive documentation for the new flexible banner property. The banner property supports simple boolean/string usage as well as advanced configuration with multiple intents (info, warning, success, error, note, launch, tip, check) and optional custom color overrides.

Key changes:

• Added detailed documentation for the new banner frontmatter property
• Documented simple usage patterns (boolean and string)
• Documented advanced usage with intents and custom colors
• Marked the beta property as deprecated with migration guidance
• Used plant-themed examples per documentation guidelines

⚠️ Important: This PR documents features implemented in fern-platform PR #4719. This docs PR should only be merged AFTER the implementation PR is merged and deployed.

Review & Testing Checklist for Human

• Verify fern-platform PR #4719 has merged - This documentation describes features that must exist in production first
• Test all banner examples - Create a test docs page and verify each example works correctly:
  • Simple: banner: true
  • Custom message: banner: "This is a test message"
  • With intent: banner: { message: "...", intent: info }
  • With custom colors: banner: { message: "...", intent: tip, color: { background: "...", text: "...", border: "..." } }
• Verify all 8 intents render correctly - Test info, warning, success, error, note, launch, tip, check intents
• Test dark mode with custom colors - Verify the warning about dark mode compatibility is accurate
• Test deprecated beta property - Verify beta: true and beta: "message" still work for backward compatibility

Test Plan

1. Create a test documentation page in your docs site
2. Add frontmatter with banner: true and verify a yellow warning banner appears
3. Change to banner: "Custom test message" and verify the custom message displays
4. Test each intent by setting banner: { message: "Test", intent: [intent_name] }
5. Test custom colors with banner: { message: "Test", intent: tip, color: { background: "#e8f5e9", text: "#1b5e20", border: "#4caf50" } }
6. Toggle dark mode and verify custom colors still look reasonable
7. Test backward compatibility by setting beta: true and verifying it still shows a banner

Notes

• Link to Devin run: https://app.devin.ai/sessions/e7e83710f40c44ffa20b715874c67eeb
• Requested by: Deep Singhvi (@dsinghvi)
• Related PR: feat(docs): add beta banner frontmatter property fern-platform#4719 (implementation)
• This PR only updates documentation; no code changes to the docs repository

[devin-ai-integration]

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