Test docs refactoring

[Youssef1313]

Summary

Describe your changes here.

Fixes #Issue_Number (if available)

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📄 File	🔗 Preview link
docs/architecture/maui/unit-testing.md	Unit testing
docs/azure/sdk/unit-testing-mocking.md	Unit testing and mocking with the Azure SDK for .NET
docs/core/testing/index.md	Testing in .NET
docs/core/testing/unit-testing-csharp-with-mstest.md	Unit testing C# with MSTest and .NET
docs/core/testing/unit-testing-csharp-with-nunit.md	docs/core/testing/unit-testing-csharp-with-nunit
docs/core/testing/unit-testing-csharp-with-xunit.md	Unit testing C# in .NET using dotnet test and xUnit
docs/core/testing/unit-testing-fsharp-with-xunit.md	Unit testing F# in .NET Core with dotnet test and xUnit
docs/core/testing/unit-testing-mstest-sdk.md	MSTest SDK overview
docs/core/testing/unit-testing-visual-basic-with-xunit.md	Unit testing Visual Basic .NET Core libraries using dotnet test and xUnit
docs/core/testing/unit-testing-with-dotnet-test.md	docs/core/testing/unit-testing-with-dotnet-test
docs/core/tools/dotnet-test.md	dotnet test
docs/csharp/language-reference/operators/null-forgiving.md	! (null-forgiving) operator (C# reference)
docs/devops/dotnet-test-github-action.md	Create a test validation GitHub workflow
docs/navigate/devops-testing/index.yml	docs/navigate/devops-testing/index
docs/navigate/devops-testing/toc.yml	docs/navigate/devops-testing/toc

[IEvangelist]

This looks awesome, thank you! One thing we might want to do as a follow up PR, is adjust the TOC to remove the "Test Platforms" top-level node, and replace it (promote) the "Microsoft Testing Platform" node. Since it's the only node under there now anyway.

[Youssef1313]

@IEvangelist I was intentionally adding MTP under "Test platforms" node as I have seen confusion multiple times around what is a test framework and what is a test platform. The ideal fix there is actually to add a second node for VSTest. Maybe we could have a node that redirects to https://github.com/microsoft/vstest

[Youssef1313]

I opened #45524 for that