QML modules: document how they should be modernized or written for new code

It seems that people are a bit confused on how to create modern QML modules (from a CMake point of view).

We should write some blogpost/documentation on how to modernize an existing Qt application from early Qt 6.X (with maybe some Qt 5 leftovers?) to have modern Qt 6.8/6.9 QML modules, and one to show how to write new QML module (targeting 6.8/6.9) for new code.

(non-exhaustive) list of things that changed:

• resource paths: :/qt/qml/ instead of root /
• setting import paths for qmlls: DEPENDENCIES TARGET ...

• getting rid of qmldir/.qrc/qmltypes files, except for qmake compatibility
• getting rid of imperative type registration in favor of declarative type registration

Maybe we could use https://github.com/emericg/QmlAppTemplate at commit fb6b0c36aa6f31372777bd1c29fa1199b07d054e as an example on how to modernize a project.

(Might need to link or refer to https://www.qt.io/blog/whats-new-for-qml-modules-in-6.5)