docs: flatten top-level project in layers example

[AloisSeckar]

🔗 Linked issue

#18954

❓ 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

As it shows up in the linked discussion, the current example for Nuxt layers may cause a bit confusion, because the prefix app/ used to identify "files in the root directory" may be understood as an actual sub-directory instead.

Simply omitting app/ in the code examples would make clear what are the docs talking about.

This is proably the only place in the docs, where such prefix is used, files are adressed as nuxt.config.ts and app.vue without anything else all around, so it will also normalize the usage.

📝 Checklist

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

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