QML/C++ integration needs better cross-linking

https://doc.qt.io/qt-6/qtqml-index.html#integrating-qml-with-c-applications currently links to two sub-pages:

• "Important C++ Classes Provided By The Qt QML Module", which contains what it states and
• "Integrating QML and C++" which is kind of a minimal tutorial and needs an overhaul (separate task).

We have however more relevant pages, at least

• the extending QML example (https://doc-snapshots.qt.io/qt6-dev/qtqml-tutorials-extending-qml-example.html)
• data type conversion (https://doc.qt.io/qt-6//qtqml-cppintegration-data.html)
• exposing attributes (https://doc.qt.io/qt-6/qtqml-cppintegration-exposecppattributes.html#exposing-properties)
• Defining QML types from C++ https://doc.qt.io/qt-6/qtqml-cppintegration-definetypes.html
• And the overview of QML/C++ integration, which link to quite a few of the above, already (https://doc.qt.io/qt-6/qtqml-cppintegration-overview.html)

We need to find a good structure (with a clean separation between proper documentation and examples/tutorials), and should also ensure that individual pages about cppintegration link back to the overview page.