Improve documentation for Next.js

[dashkevych]

At the moment, the documentation for Next.js appears somewhat confusing, making it difficult for newcomers to begin utilizing this beautiful React components library.

The current documentation features a brief section on the App Router in Next.js. Directly below this, there are three steps, each accompanied by code snippets, detailing the integration of Fluent UI with Next.js. As a result, it might be mistakenly inferred that the note about the App Router is part of the three-step instructions.

Upon further examination of the Next.js documentation, I discovered that the functions "getServerSideProps," "getStaticProps," and "getInitialProps" have been replaced by a more straightforward API in the App Router. Additionally, the App Router introduces a different folder structure, which does not include the _app and _document files.

Based on this information, I surmise that the "Note on experimental appDir support" and the three-step instructions are two distinct sections. Please correct me if I am wrong.

It would be great if you could create two well-defined sections: one for the stable version and another for the App Router. Furthermore, it would be beneficial to include a simple example in each section demonstrating the use of Fluent UI 9.