Address review comments and improve docs

[Youssef1313]

Found out that I made a commit for the branch in #45528 but forgot to push it at that time.
Cherry-picking it on top of current main :)

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Internal previews

📄 File	🔗 Preview link
docs/core/testing/microsoft-testing-platform-integration-dotnet-test.md	Use Microsoft.Testing.Platform in the VSTest mode of dotnet test
docs/core/testing/unit-testing-mstest-sdk.md	MSTest SDK overview
docs/core/testing/unit-testing-with-dotnet-test.md	Testing with 'dotnet test'

[BillWagner]

Thanks @Youssef1313

This LGTM and I'll now.