[C# 14] Build examples for extensions API reference

[BillWagner]

Describe the issue or suggestion

The new extensions feature creates new requirements for API reference docs.

Build samples that demonstrate:

• New style extension "instance" methods => Do these appear the same as existing extension methods?
• New style extension static methods => How should they appear in API reference
• New extension instance properties => How and where will they appear
• New extension static properties => How and where will they appear
• New extension operators (which are static methods) => How and where will they appear?

The API reference for IEnumerable<T> provides a template for how we document existing extension methods:

• The page for the interface, IEnumerable<T>] has a section for Extension methods.
• The extension methods do not appear in the TOC for the interface
• The class containing the extension methods, System.Linq.Enumerable provides the list of all extension methods defined in that class.
• The prototypes for those methods includes the this modifier to indicate that they extend that type.

See related issue dotnet/dotnet-api-docs#10913

More details on our first discussion can be found in this gist

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Associated WorkItem - 370943