[WIP] Refactor MTP docs

[Evangelink]

Summary

Massive refactoring of MTP docs to simplify accessing info.

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Internal previews

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📄 File	🔗 Preview link
docs/core/testing/microsoft-testing-platform-architecture-capabilities.md	Microsoft.Testing.Platform capabilities system
docs/core/testing/microsoft-testing-platform-architecture-extensions.md	Build extensions for Microsoft.Testing.Platform
docs/core/testing/microsoft-testing-platform-architecture-overview.md	Microsoft.Testing.Platform architecture overview
docs/core/testing/microsoft-testing-platform-architecture-services.md	docs/core/testing/microsoft-testing-platform-architecture-services
docs/core/testing/microsoft-testing-platform-architecture-testframework.md	Build a test framework for Microsoft.Testing.Platform
docs/core/testing/microsoft-testing-platform-architecture.md	Microsoft.Testing.Platform architecture
docs/core/testing/microsoft-testing-platform-exit-codes.md	Microsoft.Testing.Platform exit codes
docs/core/testing/microsoft-testing-platform-extensions.md	Microsoft.Testing.Platform extensions
docs/core/testing/microsoft-testing-platform-getting-started.md	docs/core/testing/microsoft-testing-platform-getting-started
docs/core/testing/microsoft-testing-platform-options.md	Microsoft.Testing.Platform configuration reference
docs/core/testing/microsoft-testing-platform-run-tests.md	Windows
docs/core/testing/unit-testing-mstest-runner-intro.md	Microsoft.Testing.Platform support in MSTest (MSTest runner)
docs/core/testing/unit-testing-mstest-sdk.md	MSTest SDK overview
docs/core/testing/unit-testing-nunit-runner-intro.md	Microsoft.Testing.Platform support in NUnit (NUnit runner)
docs/core/testing/unit-testing-with-dotnet-test.md	docs/core/testing/unit-testing-with-dotnet-test