Create tutorial for C# 14 extension members

[BillWagner]

Create a tutorial to explore C# 14 extension members.

Fixes #45612

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Internal previews

📄 File	🔗 Preview link
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs
docs/csharp/whats-new/tutorials/extension-members.md	Explore extension members in C# 14 to enhance existing types

[BillWagner]

Tagging @adegeo because he gave me some ideas for extensions

[Copilot]

Pull Request Overview

This PR creates a comprehensive tutorial for C# 14 extension members, a new language feature that enhances the existing extension methods functionality by enabling properties, operators, and type-level extensions.

Key Changes:

• Added a new tutorial demonstrating extension members syntax and capabilities
• Created supporting code samples comparing traditional extension methods with new extension member features
• Updated the table of contents to include the new tutorial

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

File	Description
docs/csharp/whats-new/tutorials/extension-members.md	Main tutorial document explaining extension members concepts and syntax
docs/csharp/toc.yml	Added navigation entry for the new extension members tutorial
docs/csharp/whats-new/tutorials/snippets/PointExtensions/*.cs	Code samples demonstrating both traditional extension methods and new extension members

[meaghanlewis]

Great tutorial @BillWagner! I left primarily style changes for you to consider.