docs: add warning for NuxtPage in app.vue (2.get-started.md)

[mdartic]

🔗 Linked issue

None

❓ 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

When I try Content for the first time, I already instantiate a nuxt app with the default template.

My app.vue was looking like that :

<template>
  <div>
    <NuxtWelcome />
  </div>
</template>

After executing what was written in the getting started, my nuxt app wasn't displaying my content/index.md page.
Instead, I was still seeing the <NuxtWelcome /> component.

I'm not sure it's useful to create an issue for this, so I create a PR directly.

📝 Checklist

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

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[farnabaz]

Thanks ❤️