documentation changes for dunfell

ping Nicholas Bennett

 
lts-5.15 release to follow dunfell instead of zeus, due to the yocto maintainers dropping their support for zeus soon.
Documentation at https://doc.qt.io/QtForDeviceCreation-5.15/qtee-custom-embedded-linux-image.html and related pages will be impacted, and the following changes need to be made: 

• References to zeus will need to be changed to dunfell ( at first glance, there are none, please validate)
• Any changes to the image build procedure or toolchain introduced by the switch to dunfell need to be integrated into the documentation.