QML registration macros are documented in QQmlEngine

They are marked with " \relates QQmlEngine", but the relation is only indirect, and they are only available when including <QQmlEngige> because that header in turn imports qqml.h, which then in turn imports qqmlintegration.h

The header which a user ideally would include is however qqmlintegration.h, given that it is the most lightweight one.

Having the documentation about registering types in a separate place from the QQmlEngine documentation also makes sense content wise, given that it's perfectly fine to use a QQmlEngine without ever having to register a type; and https://doc.qt.io/qt-6/qtqml-cppintegration-overview.html could link directly to the new, separate page specifically about the macros

QQmlEngine should still prominently link to the other place too, though, if only for SEO reasons.