Document general aspects of cross-compiling Qt

We have several pages documenting how to cross-build Qt for a specific platform. Each describes the requirements of the Qt host build in varying levels of detail:

• https://doc.qt.io/qt-6/android-building.html
• https://doc.qt.io/qt-6/ios-building-from-source.html
• https://doc.qt.io/qt-6/configure-linux-device.html#host-tools
• https://doc.qt.io/qt-6/integrity-building-qt-8155p-on-ubuntu.html#building-qt-desktop-for-cross-compiling-tools-usage
• https://doc.qt.io/qt-6/integrity-building-qt-8155p-on-windows.html#building-qt-desktop-for-cross-compiling-tools-usage
• https://doc.qt.io/qt-6/building-qt-for-qnx.html
• https://doc.qt.io/qt-6/vxworks.html#host-build
• https://doc.qt.io/qt-6/wasm.html#building-qt-from-source

A common page should describe the general aspects of cross-compiling Qt, including how to set up the Qt host build.