documentation: remove most mentions of qmlscene, use qml instead

The existence of both tools is redundant, and the intention as soon as the qml runtime was introduced (more than 5 years ago) was that it would eventually replace qmlscene. But people keep using qmlscene because of old habits, and because we keep telling them to.

Some advantages of qml are:

• can run arbitrary qml scripts, not just the ones that create an Item or a Window
• have control over whether to use QApplication, QGuiApplication or QCoreApplication (saving memory if you don't need widgets)
• the wrapping of an Item into a Window is scriptable rather than hard-coded, and thus customizable (you can even put back some of the features that good old qmlviewer had, if you like)
• you can use the shebang mechanism on Linux and macOS (start the qml file with

  #!/usr/bin/env qml
  

• convenient on Windows, macOS and other systems with graphical file managers if you make a Run association so that you can run a QML file from the file manager (that's QTBUG-33659)

In Qt 6, IMO we should remove qmlscene completely. So it's a good idea to start early with deprecating it in various ways.