Add language reference for extension indexers

[BillWagner]

Add the information for extension indexers into the language reference.

Fixes #54656

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/keywords/extension.md	Extension declaration (C# Reference)
docs/csharp/language-reference/xmldoc/examples.md	"Example XML documentation comments"
docs/csharp/whats-new/csharp-15.md	What's new in C# 15

[BillWagner]

Notes for reviewers: The extension indexers feature is included in .NET 11 preview 6. Our CI builds use .NET 11 preview 5 at the moment. Please do the first review so we can merge quickly once preview 6 is available.

[Copilot]

Pull request overview

Adds C# 15 “extension indexers” coverage to the C# language reference and supporting snippet projects, and surfaces the feature in the C# 15 “What’s new” page.

Changes:

• Adds an “Extension indexers” section to the C# 15 overview page.
• Extends the extension keyword reference to document extension indexer declarations and usage (with snippet integration).
• Updates XML doc examples/snippets to include extension indexer XML documentation and enables LangVersion=preview for the xmldoc snippet project.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-15.md	Adds a new “Extension indexers” feature section and TOC entry for C# 15.
docs/csharp/language-reference/xmldoc/snippets/xmldoc/xmldoc.csproj	Sets LangVersion to preview so the snippet project can compile with preview language features.
docs/csharp/language-reference/xmldoc/snippets/xmldoc/DocComments.cs	Adds an extension indexer example member with XML documentation.
docs/csharp/language-reference/xmldoc/examples.md	Documents cref syntax for extension indexers in the XML doc examples article.
docs/csharp/language-reference/keywords/snippets/Extensions.cs	Adds extension indexer and usage snippets to support the language-reference article.
docs/csharp/language-reference/keywords/extension.md	Adds “Extension indexers” section and updates introductory text to include indexers as valid extension members.