Document how to build Intel, Apple Silicon and universal apps for macOS

Once we ship a universal macOS Qt SDK, we should have documentation telling users how to build their projects to target either only Intel Macs, only Apple Silicon Macs, or both.

These docs should mention both qmake and CMake projects. It will probably involve Qt Creator documentation as well.

It might make sense to document limitations regarding 3rd party dependencies in user projects, and perhaps guide the users on what their options are there.