Decide what the QML Plugin Example should demonstrate

The example is rather confused, because it mixes two things:

• Actually creating a QQmlEngineExtensionPlugin
• Creating a (loadable) QML module

The first part is badly covered in the CMake example, as the example only registers a custom type,in which case we can automatically generate the plugin

The second part is somewhat discouraged, however still a necessity while e.g. image providers are tied to plugins

Lastly, we should also deduplicate the information found in https://doc.qt.io/qt-6/qqmlengineextensionplugin.html