Add closed hierarchies for C# 15, preview 5

[BillWagner]

Fixes #54013

• Add a new language reference article for the closed contextual keyword.
• Update the pattern matching reference to discuss the exhaustiveness and subsumption impact of closed hierarchies.
• Add a section in What's new for the new feature.
• Publish the feature spec.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/builtin-types/record.md	Records (C# reference)
docs/csharp/language-reference/keywords/abstract.md	"abstract keyword"
docs/csharp/language-reference/keywords/closed.md	closed (C# Reference)
docs/csharp/language-reference/keywords/index.md	"C# Keywords and contextual keywords"
docs/csharp/language-reference/keywords/sealed.md	sealed (C# Reference)
docs/csharp/language-reference/operators/patterns.md	Pattern matching - the is and switch expressions, and operators and, or, and not in patterns
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/whats-new/csharp-15.md	What's new in C# 15

[Copilot]

Pull request overview

This PR adds C# 15 Preview 5 documentation for the new closed modifier (closed class hierarchies), including a new language reference article, updated pattern matching guidance on exhaustiveness/subsumption, a “What’s new” entry, and publication wiring for the feature specification.

Changes:

• Add closed modifier reference content and link it into the keyword index and TOC.
• Extend pattern matching docs with a new “Closed hierarchy patterns” section and new snippets.
• Publish the closed-hierarchies proposal content via spec TOC and docfx.json configuration.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-15.md	Adds “Closed hierarchies” to the C# 15 preview feature list with examples and links.
docs/csharp/specification/toc.yml	Adds “Closed hierarchies” entry to the specification TOC pointing at the proposal.
docs/csharp/language-reference/toc.yml	Adds closed keyword entry to the language reference TOC.
docs/csharp/language-reference/operators/snippets/patterns/patterns.csproj	Updates snippet project TFM to net11.0 (but needs LangVersion for preview syntax).
docs/csharp/language-reference/operators/snippets/patterns/ClosedHierarchyPatterns.cs	Introduces new closed-hierarchy pattern matching snippets and narrative comments.
docs/csharp/language-reference/operators/patterns.md	Adds “Closed hierarchy patterns” section explaining exhaustiveness, accessibility, and subsumption.
docs/csharp/language-reference/keywords/snippets/shared/Closed.cs	Adds shared closed keyword snippets for the reference article.
docs/csharp/language-reference/keywords/snippets/keywords.csproj	Updates snippet project TFM to net11.0-windows (but needs LangVersion for preview syntax).
docs/csharp/language-reference/keywords/index.md	Adds closed to the contextual keywords list.
docs/csharp/language-reference/keywords/closed.md	Adds the new closed modifier reference article and links to related guidance/spec.
docfx.json	Wires in the new proposal file for build/metadata/title/description.

Comments suppressed due to low confidence (1)

docs/csharp/language-reference/keywords/snippets/keywords.csproj:12

• The keywords snippet project now targets net11.0-windows and includes C# 15 preview syntax (the closed modifier), but the project doesn't set LangVersion to preview. If the build defaults to a stable language version, the new closed snippets won't compile.

  <PropertyGroup>
    <ImplicitUsings>enable</ImplicitUsings>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net11.0-windows</TargetFramework>
    <Nullable>enable</Nullable>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <StartupObject>Keywords.Program</StartupObject>
    <GenerateDocumentationFile>False</GenerateDocumentationFile>
    <UseWPF>true</UseWPF>
  </PropertyGroup>