Unify QML's "Extending" tutorial and "Extending" examples

The tutorial (https://doc.qt.io/qt-6/qtqml-tutorials-extending-qml-example.html) basically duplicates the examples and vice versa.

We probably want to actually keep the tutorial structure, but split it into separate pages like with the example.

Moreover, the tutorial page isn't in the best shape (should promote creating a module, not a plugin, should use the tabbed UI for cmake/qmake, should link to more relevant places...)