Improve CS0430 extern alias documentation with practical usage examples

[Copilot]

The CS0430 compiler error documentation had examples that didn't demonstrate how to actually use extern aliases, making it difficult for first-time users to understand the feature. The original examples showed extern alias declarations but no practical usage.

Changes made:

1. Enhanced code examples: Added a static method M() to MyClass and demonstrated actual usage with MyType::MyClass.M() in the Main method to show how extern alias resolves naming conflicts.

2. Added cross-reference: Included a "See also" section linking to the extern alias keyword documentation to help users understand the complete picture.

Before:

// CS0430_a.cs  
public class MyClass {}  

// CS0430_b.cs  
extern alias MyType;   // CS0430  
public class Test
{  
   public static void Main() {}  // No usage shown
}  

After:

// CS0430_a.cs  
public class MyClass
{
    public static void M() { /*...*/ }
}

// CS0430_b.cs  
extern alias MyType;   // CS0430  
public class Test
{  
   public static void Main() { MyType::MyClass.M(); }  // Shows actual usage
}  

These minimal changes now clearly demonstrate how extern alias works with the :: operator and provide users with a complete understanding of the feature when encountering CS0430 errors.

Fixes #33207.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs0430.md	docs/csharp/misc/cs0430

[BillWagner]

This is ready for final review.