docs(api): add useRuntimeConfig page

[Krutie]

🔗 Linked issue nuxt/nuxt#13653

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Added API documentation for useRuntimeConfig composable

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[atinux]

Thank you ❤️

[danielroe]

Thank you! A few suggestions

[pi0]

Thanks so much for adding more api pages 💯 I have made few refactors to split the app and environment variables section.

@danielroe Would you please double check my last refactor to add back any missing comment?