Address Qt 6.8 CMake API review notes for Android aar build target

`aar` - new build target

https://codereview.qt-project.org/c/qt/qtbase/+/551535 - Android: Add facilities to build AAR packages
https://codereview.qt-project.org/c/qt/qtdoc/+/563436 - docs
https://codereview.qt-project.org/c/qt/qtbase/+/563192 - test

*Notes:*

Discussion about whether the aar should be triggered by qt_add_executable or qt_add_library
One possibility is to consider postponing to 6.9, and add as new feature for qt_add_library,
and some optin/out flag for qt_add_executable

*Action points:*
AP: Add links in docs to the upstream android documentation for AAR (and probably AAB, APK)
AP: Add a since Qt 6.8 blurb to docs, perhaps also for aab
AP: Investigate the real difference of apk vs aar, and which of the entry point commands should be used
qt_add_executable or qt_add_library