HttpServer documentation needs an overhaul

The Qt Http Server documentation was pretty bad going into the header review:

• Converters were not explained
• Parameters of functions were not explained, e.g. for the route function
• The strings for the route function was not explained
• And introduction and application on how to use it was missing

We changed some of the API and updated the documentation during header review. We tried to fix the issues above but I don't think the documentation is up to standard yet.

It would be great to have a detailed look at it and make it concise and clear.

Also, HttpServer is missing here: https://doc.qt.io/qt.html and here: https://doc-snapshots.qt.io/qt6-dev/index.html