Better syntax for HTML tab groups

See for example https://codereview.qt-project.org/c/qt/qtdeclarative/+/463298 - we often need this kind of layout in places where two or more "options" are available: qmake vs CMake, python vs C++, etc.

Apparently the tabs are only supported in html output and the resulting qdoc code is rather hard to read. There should be a better way to phrase this. Ideally, we'd find some equivalent expression of the same construct in other kinds of output and invent a unified qdoc macro to produce tabs in html and the other expression in other forms of output.