Doc: Avoid using value type names as QML IDs

https://doc.qt.io/qt-6/qtqml-documents-structure.html#valuetypebehavior explains that, when using `pragma ValueTypeBehavior: Addressable`, propoerties or IDs that are named after value types (such as "rect") become inaccessible.

Our documentation currently contains many snippets with "id: rect" (one example is at https://doc.qt.io/qt-6/qml-qtquick-propertychanges.html#details ). It would be good to rename such instances so that the examples continue working when `pragma ValueTypeBehavior: Addressable` is used.