GCStress: document how to debug GCStress failures

[BruceForstall]

We need to add a document on best practices for debugging failures in GCStress. It should cover all architectures, all platforms, and examples of different failure modes.

It might be appropriate to live in the "debugging" section:
https://github.com/dotnet/runtime/tree/master/docs/workflow/debugging/coreclr

or perhaps in the BOTR section: https://github.com/dotnet/runtime/tree/master/docs/design/coreclr/botr

cc @AndyAyersMS @janvorli

category:documentation
theme:gc-stress
skill-level:expert
cost:large