Make ButtonGroup.buttons property a default one, add examples, adjust docs

As discussed at QTBUG-95944 and associated Gerrit change https://codereview.qt-project.org/c/qt/qtdeclarative/+/366399, it may be practical in some cases to define controls / buttons in a directly in a ButtonGroup, and later use its ::buttons list property as a children value for some layout etc.

Therefore, defining buttons in a button group may and should be simplified by making group's buttons property a default one. It would also remove an extra level of indentation from user's code.

Backward compatibility concern

ButtonGroup inherits directly from a QtObject, not from an item, so it never had a default property by itself. However, if a user were to sub-type a ButtonGroup component with a custom default property (which can be declared with pure QML syntax), what would happen to their code?