IFW new example and more specific info: Offline installer with update features

Current IFW documentation states that offline installer could have update possibilities:

"Offline installers do not try to connect to an online repository at all during installation. However, the metadata configuration (config.xml) enables users to add and update components online." at https://doc.qt.io/qtinstallerframework/ifw-offline-installers.html

This is feature is often also requested by the users.

It would be helpful if an example would be created and maybe section to the offline installer instructions how exactly this needs to be implemented.