Improve extensibility documentation

[Youssef1313]

https://learn.microsoft.com/dotnet/core/testing/microsoft-testing-platform-architecture-extensions#the-idataconsumer-extensions

This document should mention how extension authors can use TestingPlatformBuilderHook to let the auto-generated code register the extension. It should also be very clear about the fact that the Guid is just a randomly generated one (without re-using a Guid from any other extension)

cc @RussKie