Create tool to inject link to Apple's online documentation in our XML documentation

[rolfbjarne]

Ref #14372, step 3

Create a command-line tool that takes the output from the C# compiler (both the xml file and the assembly), and Xcode's documentation database, and injects a link to Apple's online documentation for all the API in the assembly into the xml documentation file.

Note: the xml file created by the compiler will only contain entries for any API that has triple-slash documentation, so the tool will have to add entries for all the APIs that don't have any triple-slash documentation.

This depends on #17395.

A placeholder script has been implemented during our build: https://github.com/xamarin/xamarin-macios/blob/8efc00aa012bcff1285b829f581d43356dc7c3ac/tools/add-apple-links-to-docs.sh

This should be replaced with an invocation of the final tool.