fix: fix a section in the Next.js guide and add an example

[illright]

Background

As pointed out in the docs, it's not enough to simply have app in the root folder, you must also have pages. To ensure that the guide is working as expected, we will also provide a runnable link to a project.

Changelog

1. Changed the file structure in the App Router example to include the pages folder in the root
2. Added a StackBlitz link with a Next.js project that implements this guide

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