docs(react-native): document the Expo config plugin's scheme option

[thepushkaraj]

📋 Description

🎯 Type of Change

• 📝 Documentation update (improving existing docs)
• ✨ New documentation (adding new guides, API docs, etc.)
• 🐛 Bug fix (fixing errors, broken links, incorrect information)
• 🎨 Style/formatting (improving readability, fixing formatting)
• 🏗️ Structural change (reorganizing content, navigation updates)
• 🌐 Translation (adding or improving translations)
• 🖼️ Assets (adding or updating images, diagrams)

📖 What documentation does this PR affect?

• Getting Started / Setup
• API Reference
• Developer Resources
• Integration Guides
• Features Documentation
• External Integrations
• Changelog
• Other:

✅ Checklist

• I have read the Contributing Guidelines
• I have tested my changes locally with mintlify dev
• All links are working correctly
• Images are properly displayed and optimized
• Code examples are tested and working
• I have updated the navigation in docs.json (if applicable)
• My changes follow the project's style guide
• I have performed a self-review of my changes
• I have added appropriate frontmatter to new pages

🧪 Testing

• Verified locally at http://localhost:3000
• Checked responsive design on mobile viewport
• Tested all code examples
• Validated all external links

📸 Screenshots (if applicable)

🔗 Related Issues

Closes #

📝 Additional Notes

👥 Reviewers

@dodopayments/documentation-team