Enhance XML documentation for href attribute and br tag usage

[Copilot]

This PR enhances the XML documentation to better explain the href attribute and <br/> tag usage in C# documentation comments, addressing confusion about when and how to use these features.

Changes Made

Enhanced href attribute documentation

• Added clear explanation that href creates clickable links while cref with URLs does not
• Added practical example showing the difference between cref and href for external URLs
• Enhanced the href attribute section with guidance on when to use it vs cref

Improved <br/> tag documentation

• Added comprehensive example showing the difference between <para> (double-spaced) and <br/> (single-spaced) formatting
• Clarified when to use each tag for different spacing needs

Added working code examples

• Created HrefAndBrExamples.cs with practical examples that demonstrate the concepts
• Examples show the compiler warning (CS1584) when using cref with URLs, reinforcing why href should be used instead
• Code examples are properly tagged for snippet inclusion in documentation

Updated introduction section

• Added note explaining that HTML tags like <br/> work in IntelliSense tooltips and generated documentation
• Made HTML tag support more prominent in the overview

Example of the key difference addressed:

/// <summary>
/// This method demonstrates URL linking:
/// <see cref="https://docs.microsoft.com/dotnet/csharp"/> (won't create clickable link)
/// <see href="https://docs.microsoft.com/dotnet/csharp">C# documentation</see> (creates clickable link)
/// </summary>
public void ExampleMethod() { }

The documentation now clearly addresses the original concern that these features were available in IntelliSense but not properly documented.

Fixes #18859.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/xmldoc/recommended-tags.md	docs/csharp/language-reference/xmldoc/recommended-tags

[BillWagner]

This is a great start. I have a couple small comments.

[BillWagner]

This is now ready for final approval