This PR adds comprehensive documentation for the breaking change introduced in .NET 10 Preview 6 where HTTP3 support in System.Net.Http is disabled by default when PublishTrimmed or PublishAot is set to true.

Changes Made

• Created new breaking change document: docs/core/compatibility/networking/10.0/http3-disabled-with-publishtrimmed.md

  • Follows the established format from .github/prompts/breaking-change.md
  • Includes proper YAML front matter with metadata
  • Documents the behavior change, reasoning, and recommended actions
  • Provides clear XML configuration examples for re-enabling HTTP3 support

• Updated compatibility index: docs/core/compatibility/10.0.md

  • Added entry to the Networking section table
  • Classified as "Source incompatible" breaking change
  • Listed as introduced in Preview 6

• Updated table of contents: docs/core/compatibility/toc.yml

  • Added navigation entry under Networking → .NET 10 section
  • Maintains proper YAML structure and indentation

Documentation Content

The new documentation explains:

• Previous behavior: HTTP3 support was allowed but might not work in environments without msquic
• New behavior: HTTP3 is disabled and code is not included in trimmed/AOT applications
• Reason: Reduces application size and complexity since HTTP3 often requires extra configuration
• Recommended action: Set <Http3Support>true</Http3Support> property when HTTP3 is needed

Example configuration provided:

<PropertyGroup>
  <PublishTrimmed>true</PublishTrimmed>
  <Http3Support>true</Http3Support>
</PropertyGroup>

The documentation follows Microsoft's writing style guidelines and is consistent with other .NET 10 breaking change documents in the repository.

Fixes #47443.

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