Add preview documentation for collection expression parameters

[BillWagner]

Update pertinent articles for C# 15's collection expression parameters.

Fixes #51440

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/language-versioning.md	C# language versioning
docs/csharp/language-reference/operators/collection-expressions.md	Collection expressions - C# language reference
docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers.md	"Object and collection initializers"
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/toc.yml	Taken from https://github.com/dotnet/roslyn/wiki/Samples-and-Walkthroughs
docs/csharp/whats-new/csharp-15.md	What's new in C# 15

[BillWagner]

@adegeo I marked this ready for review even though the new examples don't build.

They should once .NET 11 preview 1 is out. I'll fix the samples and then (hopefully) merge.

[Copilot]

Pull request overview

This PR adds preview documentation for C# 15's collection expression arguments feature, which allows passing constructor or factory method arguments to collections using a with(...) element in collection expressions. The documentation is being prepared for .NET 11 and Visual Studio 2026.

Changes:

• New documentation file for C# 15 with collection expression arguments feature
• Updated collection expressions reference documentation with detailed sections on constructor arguments, collection builder arguments, and interface target types
• Updated object and collection initializers programming guide with new collection expression arguments section
• Configuration updates to TOC files, language version tables, and build configuration

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-15.md	New file introducing C# 15 with collection expression arguments feature
docs/csharp/language-reference/operators/collection-expressions.md	Added comprehensive documentation for collection expression arguments including usage examples and restrictions
docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers.md	Updated guide with collection expression arguments section and style improvements throughout
docs/csharp/language-reference/operators/snippets/shared/CollectionExpressionExamples.cs	Added code examples demonstrating collection expression arguments with custom types
docs/csharp/programming-guide/classes-and-structs/snippets/object-collection-initializers/CollectionExpressionArguments.cs	New code snippet file with collection expression arguments examples
docs/csharp/language-reference/operators/snippets/shared/operators.csproj	Updated target framework to net11.0 and added preview language version
docs/csharp/programming-guide/classes-and-structs/snippets/object-collection-initializers/object-collection-initializers.csproj	Updated target framework to net11.0 and added preview language version
docs/csharp/toc.yml	Added C# 15 entry and breaking changes link for .NET 11
docs/csharp/specification/toc.yml	Added link to collection expression arguments specification
docs/csharp/language-reference/language-versioning.md	Updated to reference C# 15 and .NET 11 support
docs/csharp/language-reference/includes/langversion-table.md	Added C# 15.0 language version entry
docs/csharp/language-reference/includes/default-langversion-table.md	Added .NET 11 mapping to C# 15
docfx.json	Added configuration for breaking changes file and collection expression arguments proposal
.openpublishing.redirection.csharp.json	Updated redirects to point to C# 15 as latest version