Add breaking change documentation for Console JSON logging duplicate messages

[Copilot]

This PR adds documentation for the breaking change introduced in .NET 10 Preview 7 that eliminates duplicate messages in Console logging when using the JSON formatter.

Changes Made

• New breaking change document: Created /docs/core/compatibility/extensions/10.0/console-json-logging-duplicate-messages.md following the established documentation format
• Updated index: Added entry to the Extensions table in /docs/core/compatibility/10.0.md
• Updated navigation: Added entry to the Extensions section in /docs/core/compatibility/toc.yml

Breaking Change Summary

Previously, Console logging with JSON formatter produced duplicate messages:

{
  "EventId": 0,
  "LogLevel": "Information", 
  "Category": "Program",
  "Message": "This is an information message.",
  "State": {
    "Message": "This is an information message.",
    "{OriginalFormat}": "This is an information message."
  }
}

After the change, the duplicate Message within the State object is eliminated:

{
  "EventId": 0,
  "LogLevel": "Information",
  "Category": "Program", 
  "Message": "This is an information message.",
  "State": {
    "{OriginalFormat}": "This is an information message."
  }
}

This behavioral change reduces log output size, improves performance, and eliminates confusion from redundant information.

Validation

• All markdown files pass linting validation (0 errors)
• YAML syntax validated for table of contents
• Document follows established breaking change format and style guidelines

Fixes #47006.

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