ApiCompatValidateAssemblies docs don't mention how to specify the baseline assemblies

[ericstj]

Issue description

I was reading the https://learn.microsoft.com/en-us/dotnet/fundamentals/apicompat/assembly-validation document and see that it tells folks how to turn on the task, but not how to provide inputs. It does mention:

Validates that there are no breaking changes across versions.

I think it will get it's multi-targeting inputs from the project automatically, but it needs the author to tell it how to find the previous version to compare to.

I think the way to do this is to specify ApiCompatContractAssembly and do so in a way that's specific to each TargetFramework of the project.

It would be nice to see a sample for how someone might do this.

cc @ViktorHofer @gewarren

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Associated WorkItem - 198657