[APICompat] Document newly added features

[ViktorHofer]

TODO:

• Document the msbuild properties that are expected to be set and are passed onto the ValidatePackage task.
• Document the ValidateAssemblies task and its parameters
• Document the Microsoft.DotNet.ApiCompat.Tool global tool and its parameters
• Document the added rules // cc @smasher164
• Consider grouping the existing PackageValidation documentation under a new APICompat section

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage

[ViktorHofer]

The target date for this is October 10th, to make sure that documentation is in place when RC2 releases publicly.

[ericstj]

Dropping some notes: https://github.com/dotnet/docs/tree/main/docs/fundamentals/package-validation is a place to put the docs. Consider predent from SDK tools / properties to see how to document this.