Add a toolchain requirement for the Qt library description for desktop

Our evaluators complain that they cannot start creating SW after installing Qt, as they assume the required toolchains will be installed as well as in some other frameworks. 

We should mention that the toolchain is needed. 

Windows:

An MSVC toolchain is required to build Qt applications. The toolchain is included in Visual Studio, which can be downloaded from Microsoft web site. 

or

A MinGW toolchain is required to build Qt applications. The toolchain is available under 'Developer and Designer Tools' in this installer. 

Linux:

A build toolchain is needed to build Qt applications in Linux. Look at https://doc.qt.io/qt-6/linux.html for installation details in your Linux distro. 

macOS:

The XCode build environment is needed to build Qt applications in macOS. You may download XCode from AppStore for free.