Add missing plugins-cache option to dotnet nuget locals documentation

[Copilot]

The documentation for dotnet nuget locals was missing the plugins-cache option that is available in newer versions of NuGet/dotnet. This option allows users to list or clear the plugins cache directory, which is used to store NuGet plugin-related data.

Changes made:

• Added plugins-cache to the list of valid CACHE_LOCATION values in the Arguments section
• Updated the description of the all option to include plugins cache
• Added example showing how to list the plugins cache directory path: dotnet nuget locals plugins-cache --list
• Added example showing how to clear the plugins cache directory: dotnet nuget locals plugins-cache -c
• Updated example descriptions to consistently mention plugins cache directory alongside other cache types

The plugins-cache option has been supported in the NuGet client since it was added to handle plugin caching functionality. This documentation update ensures users are aware of all available cache location options.

Fixes #45151.

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