This PR adds comprehensive breaking change documentation for the FilePatternMatch.Stem property nullability change in .NET 10 RC 1.

What changed

The Microsoft.Extensions.FileSystemGlobbing.FilePatternMatch.Stem property was changed from nullable (string?) to non-nullable (string) in .NET 10 RC 1. Additionally, the constructor now throws ArgumentNullException when a null value is passed for the stem parameter.

Documentation added

• New breaking change document: docs/core/compatibility/core-libraries/10.0/filepatternmatch-stem-nonnullable.md

  • Follows the official breaking change template structure
  • Includes before/after code examples showing the impact
  • Categorizes as both source incompatible and behavioral change
  • Provides clear recommended actions for affected code

• Updated index: Added entry to docs/core/compatibility/10.0.md under Core .NET libraries section

• Updated TOC: Added navigation entry to docs/core/compatibility/toc.yml

Key details covered

• Previous behavior: Constructor accepted null for stem parameter without warnings/errors
• New behavior: Constructor throws ArgumentNullException for null values, property is non-nullable
• Reason: Improves nullability accuracy since FilePatternMatch is only constructed when PatternTestResult is successful
• Recommended action: Review call sites to ensure stem cannot be null, remove unnecessary null checks

The documentation includes proper xref links to affected APIs and follows Microsoft's writing style guidelines for breaking change documentation.

Fixes #47914.

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