Improve documentation for using a TreeModel in a Qt Quick UI

Our TreeView documentation is not showing how the tree model can be created:

https://doc.qt.io/qt-6/qml-qtquick-treeview.html

Therefore, the example doesn't work (the user have to create a C++ model):
// The model needs to be a QAbstractItemModel// model: yourTreeModel
 

For reference

There is one example that uses a FileSystemModule.FileSystemModel as a model. This is defined in C++:

qtdeclarative/examples/quickcontrols/filesystemexplorer/qml/FileSystemView.qml

There's also a manual test. My hunch would be to use it's TestModel as a base of how to create a C++ tree view model for the TreeView documentation:

qtdeclarative/tests/manual/treeview/sidebyside