Document supported Qt modules on QNX page

Not all Qt modules are available on QNX. The Qt 6.8 documentation currently misses this.

Until Qt 6.7, this was documented on the central Qt Modules page. This is also the page that Qt for QNX Support - Supported Qt Modules refers to. In Qt 6.8, the (already incomplete) listing of supported platforms was removed though.

The suggestion is now to document these limitations on the respective platform pages, so for QNX directly in https://doc.qt.io/qt-6/qnx-support.html#supported-qt-modules.

There is also https://www.qt.io/product/features , but that is not versioned to a Qt release yet, and not part of the official documentation.