Address PR feedback on NuGet package README files

[Copilot]

Addresses maintainer review feedback on README documentation for NuGet packages.

Changes

• Reqnroll.CustomPlugin: Clarified description to emphasize it provides required dependencies for custom generator extensions, added link to generator plugins documentation
• Reqnroll.Microsoft.Extensions.DependencyInjection: Removed implication that Microsoft's DI container is "standard .NET", now explicitly states it's from the Microsoft.Extensions.DependencyInjection package
• Reqnroll.ExternalData: Verified documentation links are correct (no changes needed)
• Reqnroll.xUnit: Reviewed nuspec file structure against recent commits (TODO note retained for maintainer verification against published package)

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