Provide comprehensive documentation for imports in QML

We do have the general information about import paths in https://doc.qt.io/qt-6/qtqml-syntax-imports.html#qml-import-path, and various places document how qt_add_qml_module puts QML files into a place where they end up in an import path, but a less scattered style of documentation (like https://doc.qt.io/qt-6/qml-singleton.html ) would most likely be helpful.

It should not only re-iterate existing documentation, but also provide some information on when one might want to add an additional import path, how that can be done but also caution that  it's not necessary most of the time.