docs(content): fix source paths and warn in migration guide

[maximepvrt]

I think there might be an issue with the source suggested in the documentation. It doesn’t match what I have on my side, nor what’s shown in the docs (https://github.com/nuxt/ui/blob/v4/docs/app/assets/css/main.css#L4). I believe there might be an error.

🔗 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

📝 Checklist

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

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@5074

commit: 8309f24

[benjamincanac]

Thanks @maximepvrt! 😊