Update CS0518 documentation with actionable guidance for modern .NET projects

[Copilot]

The CS0518 compiler error documentation previously provided vague guidance that told developers to "make sure that the project refers to the correct mscorlib.dll" without explaining how to actually do this. This left developers without actionable steps to resolve the error, particularly in modern SDK-style projects.

What changed

• Added contextual guidance: Added a NOTE section explaining the difference between modern SDK-style projects and legacy project formats
• Updated problem description: Clarified that the root cause varies between modern and legacy project systems
• Added concrete examples: Provided specific .csproj configuration examples for both single-target and multi-target scenarios:

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

• Modernized resolution steps: Prioritized SDK-style project guidance while preserving legacy information for backwards compatibility
• Fixed formatting: Cleaned up orphaned "Optionally" section and improved overall structure

Why this matters

Modern .NET developers using SDK-style projects don't directly reference mscorlib.dll - the runtime libraries are managed automatically through the <TargetFramework> property. The previous documentation was focused on legacy .NET Framework scenarios and left modern developers without clear guidance.

The updated documentation now provides immediate, actionable steps that developers can follow to resolve CS0518 errors in their projects.

Fixes #30778.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/cs0518.md	Compiler Error CS0518

[BillWagner]

This is ready for final review.