Add breaking change documentation for .NET runtime SIGTERM signal handler removal

[Copilot]

Documents the breaking change introduced in .NET 10 Preview 5 where the runtime no longer provides default SIGTERM signal handlers on Unix systems and equivalent Windows console events.

Changes Made

• New breaking change document: docs/core/compatibility/core-libraries/10.0/sigterm-signal-handler.md

  • Comprehensive documentation following established format and guidelines
  • Includes previous/new behavior descriptions, rationale, and migration guidance
  • Provides code example showing how to replicate old behavior using PosixSignalRegistration.Create
  • Lists affected APIs: AppDomain.ProcessExit and AssemblyLoadContext.Unloading

• Updated index file: docs/core/compatibility/10.0.md

  • Added entry in Core .NET libraries table for proper categorization

• Updated table of contents: docs/core/compatibility/toc.yml

  • Added navigation entry for the new document

Key Information

• Version: .NET 10 Preview 5
• Type: Behavioral change
• Impact: Applications relying on automatic SIGTERM handling may not receive ProcessExit/Unloading events
• Mitigation: Use higher-level APIs (ASP.NET Core, UseConsoleLifetime) or implement custom signal handlers

The documentation helps developers understand this breaking change and provides clear guidance on how to adapt their applications.

Fixes #46226.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/core-libraries/10.0/sigterm-signal-handler.md	.NET runtime no longer provides default SIGTERM signal handler
docs/core/compatibility/toc.yml	docs/core/compatibility/toc

[jkotas]

LGTM!