Add Xml docs tutorial

[BillWagner]

Fixes #27639

Add a tutorial for XML documentation that takes the previous Object oriented tutorial and adds XML comments.

I also updated some of the code constructs in the related preceding tutorials.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/tutorials/classes.md	Explore object oriented programming with classes and objects
docs/csharp/fundamentals/tutorials/oop.md	"Object-Oriented Programming"
docs/csharp/fundamentals/tutorials/xml-documentation.md	Tutorial: Create XML documentation

[Copilot]

Pull Request Overview

This PR adds a comprehensive XML documentation tutorial to the C# fundamentals section and modernizes existing code examples. The tutorial teaches developers how to create effective XML documentation comments using the object-oriented programming sample as a foundation.

• Introduces XML documentation concepts and demonstrates practical implementation
• Updates Transaction class to use modern record syntax across multiple tutorials
• Refreshes dates and language in existing object-oriented programming tutorials

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/csharp/fundamentals/tutorials/xml-documentation.md	New tutorial covering XML documentation comments with practical examples
docs/csharp/fundamentals/tutorials/snippets/xml-documentation/*.cs	Complete code examples with comprehensive XML documentation
docs/csharp/fundamentals/tutorials/snippets/*/transaction.cs	Modernized Transaction class to use record syntax
docs/csharp/fundamentals/tutorials/oop.md	Updated language and dates for clarity
docs/csharp/fundamentals/tutorials/classes.md	Updated language and dates for clarity

[IEvangelist]

This looks good to me. Is there any value in adding screen shots of rendered XML comments on APIs to help visualize the result?