Update .NET 10 "What's New" documentation for RC 1 release

[Copilot]

This PR updates the .NET 10 "What's New" documentation to reflect the RC 1 release, incorporating new features and API changes introduced in this release candidate.

Key Changes

Updated Release Information

• Updated all four documentation files (overview.md, runtime.md, libraries.md, sdk.md) to reference RC 1 as the latest update
• Updated ms.date metadata to 09/09/2025 (RC 1 release date)
• Maintained consistent ai-usage: ai-assisted metadata across all files

Enhanced Post-Quantum Cryptography Documentation

• Added ML-DSA External Mu signature support with detailed code examples showing SignMu() and VerifyMu() methods
• Updated experimental status information: MLKem type is no longer marked as [Experimental] as of RC 1
• Clarified that MLDsa will follow a similar path with experimental designation moving from the entire type to specific methods

New UTF-8 Hex-String Conversion APIs

• Added comprehensive documentation for new UTF-8 support in System.Convert class
• Documented new methods: FromHexString(), TryToHexString(), and TryToHexStringLower() overloads that work with UTF-8 byte sequences
• Explained performance benefits of avoiding intermediate string allocations

Tensor API Stabilization

• Updated documentation to reflect that tensor APIs are now stable and no longer experimental in RC 1
• Maintained note about requiring the System.Numerics.Tensors NuGet package
• Emphasized integration with C# 14 extension operators for arithmetic operations

Quality Assurance

• All changes pass markdown linting with 0 errors
• xref links use proper format and reference valid API documentation IDs
• Code examples follow established patterns and are properly formatted
• Content adheres to Microsoft Writing Style Guide requirements
• Documentation structure maintains consistency with existing patterns

The documentation now accurately reflects the current state of .NET 10 RC 1, providing developers with up-to-date information about new features and API changes.

Fixes #48315.

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Internal previews

📄 File	🔗 Preview link
docs/core/whats-new/dotnet-10/libraries.md	What's new in .NET libraries for .NET 10
docs/core/whats-new/dotnet-10/overview.md	What's new in .NET 10
docs/core/whats-new/dotnet-10/runtime.md	What's new in .NET 10 runtime
docs/core/whats-new/dotnet-10/sdk.md	What's new in the SDK and tooling for .NET 10

[BillWagner]

I had one wording suggestion.

[BillWagner]

This is ready for a second review

[BillWagner]

@gewarren Should we wait to merge this until the API reference updates are merged?

[BillWagner]

closing and reopening for a fresh CI build.