Address Qt 6.8 CMake API review notes for QT_ANDROID_GENERATE_JAVA_QML_COMPONENTS

https://codereview.qt-project.org/c/qt/qtbase/+/562130 - Android: Add variable to opt-in QtQmlComponent code generation

*Notes:*

This is currently a project based approach, and not a target based-approach.

*Action points:*

AP: Missing backlink to `QT_QML_GENERATE_JAVA_CLASS`.
AP: Add a doc note that enabling this variable makes sense only for AAR packages
AP: Fabian: Does it makes sense that is is project-based approach instead of target?
AP: Clarify in the docs that it is a boolean
AP: Add a doc link to whatever `QtQmlComponent` is
AP: Add example snippet to doc
AP: The name 'QtQmlComponent' will be changed to something, so have to adjust the docs here too
AP: Keep it TP for now, and confirm after we talk with customers who need this and PMs (sync with Mr. Bruno)