Address Qt 6.8 CMake API review notes for qtdeclarative QTP0004

New policy to generate qmldir files in each qml module's subdirectory.

https://codereview.qt-project.org/c/qt/qtdeclarative/+/551058 - Generate qmldir files for extra directories with QML files (new policy)

https://doc-snapshots.qt.io/qt6-dev/qt-cmake-policy-qtp0004.html
https://doc-snapshots.qt.io/qt6-dev/qt-add-qml-module.html#auto-generating-qmldir-and-typeinfo-files

*Action points:*
AP: Fix examples to use new policy
AP: Fix or silence policy warnings in qtdeclarative modules like `quickdialogsquickimpl`
AP: Add a example snippet to the docs
AP: Fabian: Explain a bit more when you would want to use this, and the benefit, especially for examples, into the policy docs