Consolidate, modernize, and fix docs on using QML plugins that contain C++ code

See:

• https://doc.qt.io/qtcreator/creator-qml-modules-with-plugins.html
• https://doc.qt.io/qtdesignstudio/creator-qml-modules-with-plugins.html

Problems

1. These are 2 slightly different versions of the same thing. They should either be kept in sync, or refactored and cross-referenced to avoid duplication
2. The Qt Design Studio docs talk about adding commands to qmake/CMake files to use the modules. This is wrong, as Qt DS uses .qmlproject files, not .pro or CMakeLists.txt files
3. The docs still instruct readers to create qmldir files. This is outdated, as we should usually be relying on CMake's qt_add_qml_module() to generate them nowadays (see https://www.qt.io/blog/introduction-to-the-qml-cmake-api )