Allow classes to document std::format support

To document std::format support for various Qt types, it might make sense to provide a qdoc macro that would expand into something like "This class supports formatting with std::format".
Update: it would be nice if the generated text could link to some "std::format support in Qt overview", once it's provided (see QTBUG-128837).

Ideally, the macro should also allow to add custom text, i.e. the actual format string specifications.

Maybe we could have something like compareswith/endcompareswith pair?

I'd suggest:

\stdformattable
Arbitrary text can be added here
\endstdformattable