Refresh testing docs

[Youssef1313]

Summary of the changes:

• Restructures the ToC
  • MTP and the test frameworks are now not at the same level. Having them at the same level is misleading. The test frameworks are now grouped under "Test frameworks" node, and MTP is added under "Test platforms" node.
• nit: Rename xUnit header to xUnit.net in testing/index.md
• Enrich the documentation for Testing tools section in testing/index.md
  • Instead of talking right away about test frameworks in that section. It's now divided into two parts. The test platforms and test frameworks. The test platforms gives a quick history overview and mentions MTP. It wasn't ideal that testing/index.md didn't mention MTP at all. As that page is expected to be the first page for users looking into information related to testing.
  • For each major frameworks, there is now a link to enabling MTP support.
• Improve MSTest runner page
  • Clarify that page by making "Microsoft.Testing.Platform" more mainstream in wording and title.
  • Avoid mentioning a preview version.
  • Add TestingPlatformDotnetTestSupport as part of the steps to enable MTP. I have seen projects in reality that enabled MTP only by EnableMSTestRunner, which seems to be like they followed the documentation. So they are not completely migrating to MTP due to that.
• Update NUnit runner page with similar changes as MSTest runner page
• Add xUnit official doc for MTP in ToC

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Internal previews

📄 File	🔗 Preview link
docs/ai/conceptual/evaluation-libraries.md	The Microsoft.Extensions.AI.Evaluation libraries (Preview)
docs/core/testing/index.md	Testing in .NET
docs/core/testing/unit-testing-mstest-runner-intro.md	Microsoft.Testing.Platform support in MSTest (MSTest runner)
docs/core/testing/unit-testing-nunit-runner-intro.md	docs/core/testing/unit-testing-nunit-runner-intro
docs/navigate/devops-testing/toc.yml	docs/navigate/devops-testing/toc

[Youssef1313]

@gewarren @IEvangelist Could I get a review here please? Thanks!

Note: to better understand the ToC changes, you can turn off whitespace changes.

[Youssef1313]

Thanks so much for the review @gewarren and for the suggested improvements! I addressed all the comments. Is it good to merge now?