As a user of Qt, I'd like to have an overview over which classes already support C++20 op<=>

Each individual class that supports the spaceship operator in C++20 builds should already mention this in the docs (needs confirmation).

The reverse is not true: we don't have a list of Qt types that support the spaceship operator. Esp. in Qt 6.8 and 6.9, where not all classes have been ported, yet, such a list would be good starting point for users that wish to learn about C+20 support, and could be linked from the C+20 overview page, too (cf. QTBUG-128837).

Acceptance criteria:

• add a new \group, say three_way_comparables
• add each \class that has a spaceship operator in C++20 builds to that group
• display the group in a suitable location
  • link that location from the C++20 overview page (cf. QTBUG-128837)

The "suitable location" is probably not the C++20 overview page itself, because that list will become long, so it should probably be a separate page, akin to https://doc.qt.io/qt-6/datastreamformat.html