Offline ibl prefiltering needs better docs, esp. CMake integration

We added minimal documentation in 6.1 when balsam gained the ability to do the ibl prefiltering offline: https://doc.qt.io/qt-6/quick3d-asset-conditioning-ibl.html#pre-generating-ibl-cubemap

Whereas the CMake integration is not documented at all, except in the CMake Macros file itself.

This is not ideal. See for example https://lists.qt-project.org/pipermail/interest/2021-August/037352.html where the users would have been better served by documenting and promoting the offline baking more prominently.

Check the discussion and status on QTBUG-95179 before this.