Make qdoc recognise and document technology preview APIs

Once https://codereview.qt-project.org/c/qt/qtbase/+/536809 lands, we can declare APIs (classes, member functions, enums, free functions) as technology preview in our headers, so that we can a) see that they are in TP state when reviewing headers, and b) notice when they move out of TP.

An additional benefit is that we can implicitly document respective APIs using consistent language - once qdoc knows about the tag. That should ideally make it obsolete to use \preliminary and equivalent tags explicitly in the respective documentation, although a useful first step might also be to warn if APIs marked with the tag don't use the correct existing tag (depends on which is easier to implement).