Add a How To page for Qt Quick topics

A user was asking how to have a Button that, when clicked, did something in C++. I started looking for documentation that I could share with them.

I found https://doc.qt.io/qt-6/qtdoc-tutorials-alarms-example.html, which is too complex.

I thought https://doc.qt.io/qt-6/qtqml-cppintegration-overview.html might have something, but it doesn't have any concrete examples.

I shared https://www.youtube.com/watch?v=9BcAYDlpuT8, which is too thorough an explanation for such a simple question.

I thought the QML book might have something, but the closest thing it has mentions setContextProperty (https://www.qt.io/product/qt6/qml-book/ch18-extensions-qml-runtime), which is not best practice.

We should add a page of common tasks with straight-forward solutions, as we have for Creator and Qbs:

• https://doc.qt.io/qtcreator/creator-how-tos.html
• https://doc.qt.io/qbs/howtos.html