Add breaking change documentation for dotnet new sln defaulting to SLNX format

[Copilot]

This PR adds comprehensive breaking change documentation for the behavioral change where dotnet new sln defaults to creating SLNX-format solution files instead of SLN-format files in .NET 10 RC 1.

Changes Made

• Created new breaking change document at docs/core/compatibility/sdk/10.0/dotnet-new-sln-slnx-default.md following the official template structure
• Updated the compatibility index (docs/core/compatibility/10.0.md) to include the new breaking change in the SDK and MSBuild section table
• Updated the table of contents (docs/core/compatibility/toc.yml) to include proper navigation for the new document

Documentation Details

The breaking change document includes:

• Clear description of the behavioral change from SLN to SLNX format
• Code examples showing both previous SLN format and new SLNX format
• Proper categorization as a behavioral change with link to categories
• Justification explaining the benefits of SLNX format and its stability since 9.0.200
• Recommended action for users who still need SLN format: dotnet new sln --format sln
• Appropriate frontmatter with AI disclosure and issue reference

Both the index table and TOC entries are placed in alphabetical order to maintain consistency with existing documentation structure.

Fixes #48192.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/sdk/10.0/dotnet-new-sln-slnx-default.md	dotnet new sln defaults to SLNX file format
docs/core/compatibility/toc.yml	docs/core/compatibility/toc
docs/core/tools/dotnet-sln.md	dotnet sln

[baronfel]

LGTM - and thank you for also changing the dotnet sln docs to match!