Modernize docs for creating QQmlEngineExtensionPlugin

According to https://doc.qt.io/qt-6/qqmlengineextensionplugin.html#details, the first step is to subclass QQmlEngineExtensionPlugin and add the Q_PLUGIN_METADATA() macro.

However, doing this together with the latest CMake QML API will cause errors:

multiple definition of `qt_plugin_query_metadata'
multiple definition of `qt_plugin_instance'

This is because qt_add_qml_module() + qt_add_library() already auto-generates a QQmlEngineExtensionPlugin subclass for the library, so the developer should not do this manually.

Notes

To see the auto-generator in action, simply use Qt Creator 8.0 to generate a Hello World plugin: "File" > "New Project..." > "Library" > "Qt Quick 2 Extension Plugin"