Improve helper text around macOS third-party package merging

[freakboy3742]

What is the problem or limitation you are having?

When deploying a dependency on macOS, platform-specific binaries are merged into a universal form. However, there are sometimes discrepancies between platforms in some files - especially .h, .a, and .py files used for configuration - e.g.,

These messages can usually be ignored - but it's not clear from Briefcase's output that this is the case.

Describe the solution you'd like

• A reference in the macOS platform quirks that describes what these messages are warning about
• If a problem file is found, a generic message message describing the problem should be printed, directing attention to the quirks page. i.e., output should be something like:

Thinning libraries in app_packages.x86_64... 
   ---------------------------------------- 100.0%
Merging app_packages.x86_64 ... done

Discrepancies were found between the x86_64 and arm64 versions of some files. 
The x86_64 version of the files will be used; you should confirm whether these 
discrepancies will cause a problem at runtime. For more details, see:

    https://briefcase.readthedocs.org/...

The following files are affected:

* numpy/__config__.py
* numpy/core/lib/libnpymath.h
...

For a really smooth user experience, it may be desirable to:

• Allow for a user-specified "allowlist" of files that have known but acceptable discrepancies
• Files that would be removed by the cleanup_paths marker should not be reported as a problem

Describe alternatives you've considered

We could potentially maintain an internal allowlist of known acceptable discrepancies. This would help avoid common packages (like numpy) raising this warning. However, that then puts the onus on us to do compatibility testing.

We could explicitly exclude some file types, like .h and .a, as these files are highly unlikely to be used at runtime. However, unlikely doesn't mean impossible.

Additional context

Arose through the discussion on #1972.