Add the doc for newly added publish-related property PublishReferencesSymbols

[GangWang01]

Summary

dotnet/sdk#49707 added a new publish-related property PublishReferencesSymbols. Add the corresponding doc.

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Internal previews

📄 File	🔗 Preview link
docs/core/project-sdk/msbuild-props.md	MSBuild reference for .NET SDK projects

[Copilot]

Pull request overview

This PR adds documentation for the newly introduced PublishReferencesSymbols MSBuild property, which was added to the .NET SDK in dotnet/sdk#49707. The documentation follows the established pattern for similar publish-related properties in the file.

• Adds PublishReferencesSymbols to the table of contents in alphabetical order
• Documents the property's behavior for copying symbol files (PDB files) during publish
• Specifies the default value as true

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[Copilot]

[nitpick] For consistency with the similar property PublishReferencesDocumentationFiles (line 529), consider adding the clarifying phrase "instead of just run-time assets like DLL files" to explain what normally gets copied. The full sentence would read: "When this property is true, symbol files (also known as PDB files) for the project's references are copied to the publish directory, instead of just run-time assets like DLL files."

Suggested change

	When this property is `true`, symbol files (also known as PDB files) for the project's references are copied to the publish directory. This property defaults to `true`.
	When this property is `true`, symbol files (also known as PDB files) for the project's references are copied to the publish directory, instead of just run-time assets like DLL files. This property defaults to `true`.