Update docs (landing page etc)

[rcj1]

Summary

These changes mostly affect the landing page of the diagnostics documentation.

• Added a paragraph about profilers as well as a summary page directing people to VS and dotnet-trace.
• Moved "writing a custom diagnostic tool" down on the page as it seems like a less common case.
• Used more general title.
• Removed "unit testing" from the front page as it is a different topic that is already well linked to.
• Moved "resource monitoring" to the "metrics" section, similar to other built-in metrics.
• Moved "collect diagnostics in containers" to the bottom of the page in the specialized diagnostics overview.

Fixes dotnet/diagnostics#3039

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Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/index.md	Diagnostics in .NET
docs/core/diagnostics/profilers.md	Profiling tools in .NET
docs/core/diagnostics/specialized-diagnostics-overview.md	Specialized diagnostics
docs/navigate/tools-diagnostics/index.yml	[linkListType: architecture
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc

[noahfalk]

This looks good to me! I imagine the docs crew will want to take a look as well.

[noahfalk]

Demoted "resource monitoring" to "metrics" from the top-line index as it is largely deprecated.

Resource Monitoring as a whole is not deprecated. The only part that is deprecated is the particular IResourceMonitor API. Moving it off the main page was just intended to bucket it with other Metrics related information. We don't give any other built-in metrics dedicated space on the overview page so I don't think there is reason to give resource monitoring metrics extra focus.

I went ahead and edited the PR description to avoid causing confusion about the state of the resource monitoring feature.

[gewarren]

I left some mostly style related suggestions.