qdoc: Allow modules to explicitly declare their state

Qt modules have a life-time: Most start with a 'technical preview', then are promoted to a fully supported module, before (potentially) being 'deprecated', and eventually removed.

The documentation should be more explicit when modules are being in 'technical preview'. Right now, the state is only listed on https://doc.qt.io/qt-6/qtmodules.html, but doesn't show up in the module overview, or the class documentation. One suggestion to fix this is to add a

\modulestate "Technical Preview"

And show appropriate warnings in all generated documentation pages:

breadcrumb menu could say e.g. "Qt 6.5 > QtHttpServer (Technical Preview)"

https://doc-snapshots.qt.io/qt6-dev/qthttpserver-module.html
could say "This module was introduced in Qt 6.4, and is in Technical Preview state."