feat(nuxi): warn when prerendering routes with ssr: false

[danielroe]

🔗 Linked issue

resolves #12393

❓ 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

We can warn when users have ssr: false enabled but are running nuxi generate. This is almost always the wrong choice although it may be desirable in a small subset of cases.

I've also updated the documentation to make this clearer.

📝 Checklist

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

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