Add breaking change documentation for project.json removal from dotnet restore

[Copilot]

This PR adds comprehensive breaking change documentation for the removal of project.json support from dotnet restore in .NET 10 Preview 7.

Changes Made

New Breaking Change Document: Created complete documentation following the standard Microsoft breaking change format at /docs/core/compatibility/sdk/10.0/dotnet-restore-project-json-unsupported.md including:

• Clear description of the breaking change behavior
• Proper categorization as a "Source incompatible" change
• Historical context about project.json deprecation since 2017
• Migration guidance pointing to legacy dotnet migrate command and PackageReference format
• Reference to official NuGet migration documentation

Updated Documentation Index: Added the new breaking change to the main compatibility table in /docs/core/compatibility/10.0.md under the SDK and MSBuild section with proper ordering and categorization.

Updated Table of Contents: Added the new entry to /docs/core/compatibility/toc.yml in the appropriate alphabetical position within the SDK and MSBuild section for .NET 10.

Validation

• ✅ Follows Microsoft documentation style guidelines from .github/copilot-instructions.md
• ✅ Uses the breaking change template from .github/prompts/breaking-change.md
• ✅ Markdown syntax validated with markdownlint-cli2
• ✅ All internal links verified to exist
• ✅ Consistent formatting with existing breaking change documentation

The documentation properly explains that starting with .NET 10 Preview 7, dotnet restore will ignore project.json based projects rather than restoring their dependencies, completing the transition away from the format that was deprecated in 2017 and replaced by PackageReference.

Fixes #47968.

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