Add new dotnet tool exec command documentation and point existing docs to use dnx as the top-level method

[baronfel]

Summary

Fixes #46799

A subsequent PR will add the new RID-specific tools and will more drastically change the overall tools content, but I wanted to get the more straightforward changes in first.

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Internal previews

📄 File	🔗 Preview link
docs/core/diagnostics/dotnet-counters.md	docs/core/diagnostics/dotnet-counters
docs/core/tools/dotnet-tool-exec.md	docs/core/tools/dotnet-tool-exec
docs/core/tools/global-tools-how-to-use.md	docs/core/tools/global-tools-how-to-use
docs/core/tools/local-tools-how-to-use.md	docs/core/tools/local-tools-how-to-use
docs/navigate/tools-diagnostics/toc.yml	docs/navigate/tools-diagnostics/toc

[gewarren]

Thanks for adding this.