docs(input): add example for maxlength

[vanngoh]

🔗 Linked issue

❓ 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)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Added an example for maxlength and how to set the trailing character counter in <UInput>

I think this would help those developers who are new to NuxtUI to onboard easily.

📝 Checklist

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