Add language reference for union types (C# 15)

[BillWagner]

Contributes to #52320

This PR handles the language reference and What's new changes. The fundamentals changes will be added later.

1. Publish the unions.md speclet as part of the overall specification.
2. Add unions reference and sample.
3. Refactor sample.
4. Edit reference article.
5. Edit other existing language reference articles to refer to union types.
6. Update TOC and other links.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/builtin-types/union.md	Union types (C# reference)
docs/csharp/language-reference/builtin-types/value-types.md	"Value types"
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/operators/switch-expression.md	docs/csharp/language-reference/operators/switch-expression
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/tour-of-csharp/tips-for-javascript-developers.md	Roadmap for JavaScript and TypeScript developers learning C#
docs/csharp/whats-new/csharp-15.md	What's new in C# 15

[Copilot]

Pull request overview

Adds C# 15 union types coverage to the C# language reference and related “What’s new”/cross-linking docs, and publishes the unions speclet in the rendered proposal set.

Changes:

• Adds a new C# reference article for union types plus a full runnable snippet set.
• Updates C# 15 “What’s new” and other language reference pages to reference unions and union exhaustiveness in pattern matching and switch expressions.
• Wires the unions speclet into docfx and TOCs.

Reviewed changes

Copilot reviewed 21 out of 21 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 “Union types” section and updates C# 15 intro text and links.
docs/csharp/tour-of-csharp/tips-for-javascript-developers.md	Updates JS/TS comparison to note unions are supported starting C# 15.
docs/csharp/specification/toc.yml	Adds unions proposal to the specification TOC.
docs/csharp/language-reference/toc.yml	Adds “Union types” entry to language reference TOC.
docs/csharp/language-reference/operators/switch-expression.md	Adds union-specific exhaustiveness guidance and tweaks heading.
docs/csharp/language-reference/operators/patterns.md	Adds union patterns section and refines wording in multiple pattern sections.
docs/csharp/language-reference/builtin-types/value-types.md	Adds union types to the “kinds of value types” list.
docs/csharp/language-reference/builtin-types/union.md	New reference article describing union declarations, matching, nullability, and manual patterns.
docs/csharp/language-reference/builtin-types/snippets/unions/unions.csproj	New snippet project for union type examples (net11/preview).
docs/csharp/language-reference/builtin-types/snippets/unions/Program.cs	Entry point running all union snippet scenarios.
docs/csharp/language-reference/builtin-types/snippets/unions/BasicUnion.cs	Snippets for basic union declaration, conversions, and pattern matching.
docs/csharp/language-reference/builtin-types/snippets/unions/GenericUnion.cs	Snippets for generic union (Option<T>) examples.
docs/csharp/language-reference/builtin-types/snippets/unions/ValueTypeCases.cs	Snippets for unions with value-type cases.
docs/csharp/language-reference/builtin-types/snippets/unions/BodyMembers.cs	Snippets showing additional members in a union declaration body.
docs/csharp/language-reference/builtin-types/snippets/unions/NullHandling.cs	Snippets demonstrating null handling and nullable union usage.
docs/csharp/language-reference/builtin-types/snippets/unions/ManualUnion.cs	Snippets for manually implemented union types via attribute/interface pattern.
docs/csharp/language-reference/builtin-types/snippets/unions/NonBoxingAccess.cs	Snippets for non-boxing access pattern (HasValue/TryGetValue).
docs/csharp/language-reference/builtin-types/snippets/unions/ClassUnion.cs	Snippets for class-based union type implementation and matching.
docs/csharp/language-reference/builtin-types/snippets/unions/MemberProvider.cs	Commented-out draft snippet for future “member provider” support.
docs/csharp/language-reference/builtin-types/snippets/unions/RuntimePolyfill.cs	Adds a local polyfill for UnionAttribute/IUnion for preview runtimes.
docfx.json	Includes the unions proposal in proposal publishing and metadata maps.

[MadsTorgersen]

Looks great!