docs: add examples for index.ts files on server routes

[filipweidemann]

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

While getting to know how Nuxt handles server routes I stumbled over the required naming of the .ts files to create a rather common API structure:

• /api/project/
• /api/project/[id]
• /api/project/[id]/invite

The docs didn't convey that handling the "root" namespace of you API requires setting one (or multiple) index.ts files inside of the project directory so that /api/project will be handled correctly and with potentially multiple HTTP verbs and different logic.

This is obviously a quite common technique in all kinds of module-situations in the JS world but I thought it would be cool to include this as a short and concise example alongside the existing docs about the file router.

I am also aware that many things are more or less directly related to nitro and may or may not need to be directly documented inside the Nuxt docs but since this documentation is the first contact point for everyone learning Nuxt I though that I could help to make this more clear for all future learners!

If you have any question or opinion about this, please ask straight away. I'd love to make this even better if needed. :)

📝 Checklist

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

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[filipweidemann]

Thanks @Atinux. I committed your suggestions.