Update(docs): Edit Next.js file example to use the new app router

[AhmedBaset]

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See this qoute:

if you use [CodeSandbox](https://codesandbox.io/) 
or [Create React App](https://create-react-app.dev/), 
the root component is defined in `src/App.js`. 
If you use the framework [Next.js](https://nextjs.org/), 
the root component is defined in `pages/index.js`.

This PR edits pages/index.js to be app/page.ts

[github-actions[bot]]

Size changes

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This PR introduced no changes to the JavaScript bundle! 🙌