docs: add cdn proxy section

[clemcode]

❓ 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

Some Cloudflare proxy options can lead to unexpected behavior for Nuxt. This PR integrates in the 3.x documentation the instructions documented on the 2.x docs

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[danielroe]

It feels like maybe this should be on a provider-specific page, maybe in the Nitro docs? Or, if there is more content like this in the docs, perhaps we could assemble it into a page on 'deployment caveats' or similar?

[clemcode]

I agree this is not an ideal place. Is it possible it also affects Nitro independently of Nuxt? If so, happy to make the PR on the Nitro repository.
For now, it is the only caveat of this type that I witnessed, but if more of these should arise, a dedicated page on the Nuxt docs seems the way to go.

[Atinux]

I could move this later in a deployment best practices / caveats but I think right now it is already good to have this in docs