Document IActionContextAccessor deprecation breaking change for .NET 10

[Copilot]

This PR documents the breaking change announced in aspnet/Announcements#520 regarding the deprecation of IActionContextAccessor and ActionContextAccessor in ASP.NET Core for .NET 10 Preview 7.

Changes Made

• Created new ASP.NET Core 10.0 compatibility directory structure
• Added comprehensive breaking change documentation for the IActionContextAccessor deprecation (diagnostic ID ASPDEPR006)
• Updated the main 10.0.md index file to include the new breaking change in the ASP.NET Core section
• Updated toc.yml to include the new documentation in the navigation structure

Documentation Details

The breaking change documentation includes:

• Clear explanation of why IActionContextAccessor is being deprecated in favor of endpoint routing
• Migration guidance showing how to replace IActionContextAccessor with IHttpContextAccessor and HttpContext.GetEndpoint()
• Before/after code examples demonstrating the migration pattern
• Proper categorization as both source incompatible and behavioral change
• API references using the correct xref format for the affected types

The documentation follows the established format and style guidelines, including proper markdown formatting and all linting checks pass successfully.

Fixes #47619.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/aspnet-core/10/iactioncontextaccessor-obsolete.md	IActionContextAccessor and ActionContextAccessor are obsolete
docs/core/compatibility/toc.yml	docs/core/compatibility/toc