Clarify Quick Controls and Quick Template

Whatever listed here:
https://doc.qt.io/qt-6/qtquick-templates-qmlmodule.html
eventually leads to Control. But actually, Template is not the same as Control. The former is more like "invisible metadata" where the latter is the actual visible UI components.

I think people use Control in most of cases and Template may not be that relevant in actual production. But it is probably still better to clarify it in docs.