qdoc: Provide a standard way to prominently highlight that a QML type is a singleton

Examples:

• https://doc.qt.io/qt-6/qml-qtqml-qt.html
• https://doc.qt.io/QtApplicationManager/qml-qtapplicationmanager-systemui-notificationmanager.html
• https://doc.qt.io/qt-6/qml-qtquick-inputmethod.html

The types above are QML singletons, but the "brief" at the top of the type reference does not mention this fact at all. Rather, the reader must scroll down to the Detailed Description to find out that it's a singleton. This can be easy to miss.

The information is a bit more prominent at https://doc.qt.io/QtApplicationManager/qml-qtapplicationmanager-intentclient.html because the first word in the brief is "Singleton".