Tutorial: Choosing between tuples, records, structs, and classes

[BillWagner]

Fixes #51996

This pull request introduces a new tutorial to the C# Fundamentals documentation that helps developers choose between tuples, records, structs, classes, and interfaces by walking through a practical coffee shop example. It includes a comprehensive, example-driven guide, a complete code sample, and updates the table of contents to surface the new material.

New tutorial and code sample:

• Added a detailed tutorial, choosing-types.md, that explains when to use tuples, record classes, record structs, classes, and interfaces, with practical examples and a decision guide table.
• Introduced a full runnable code sample in snippets/choosing-types/Program.cs that demonstrates each type in context, including tuples, records, structs, classes, and interfaces.

Documentation navigation:

• Updated the table of contents (toc.yml) to add the new tutorial under the Fundamentals section for easier discovery.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/tutorials/choosing-types.md	Tutorial: Choose between tuples, records, structs, and classes
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs