Set up and write QQEM documentation

The Qt Quick Effect Maker is a new product and need documentation.

We need to plan the content, set up the infrastructure, and write the documentation.

There is some existing documentation:

• https://www.qt.io/blog/introducing-qt-quick-effect-maker
• https://www.qt.io/blog/creating-an-effect-with-qqem
• https://doc.qt.io/qtdesignstudio/qt-effect-maker.html
• In-app documentation

Documentation structure draft