Add breaking change documentation for preserving null values in configuration

[Copilot]

This PR adds comprehensive breaking change documentation for .NET 10 Preview 7, covering the preservation of null values in configuration binding. Previously, the configuration binder treated null values as missing values and skipped binding them. The JSON configuration provider also converted null values to empty strings, causing confusion for developers.

Changes Made

New Documentation

• docs/core/compatibility/extensions/10.0/configuration-null-values-preserved.md - Complete breaking change documentation following the established template

Updated Navigation

• docs/core/compatibility/toc.yml - Added TOC entry for the new breaking change
• docs/core/compatibility/10.0.md - Added entry to the Extensions section table

Key Features

The documentation includes:

• Clear before/after examples showing how the behavior changed for both JSON and in-memory configuration providers
• Validated C# code samples that compile successfully and demonstrate the breaking change
• Proper API references using xref format for affected configuration binding methods
• Migration guidance for users who prefer the previous behavior
• Correct categorization as a behavioral change affecting .NET 10 Preview 7

Example of the Change

Previous behavior (null converted to empty string):

StringProperty: '', intProperty: 123
Array1: '', ''

New behavior (null values preserved):

StringProperty: 'null', intProperty: null
Array1: 'null', 'null'

This change makes configuration binding more intuitive and consistent with developer expectations when working with explicit null values in configuration.

Fixes #46890.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/extensions/10.0/configuration-null-values-preserved.md	Null values preserved in configuration
docs/core/compatibility/toc.yml	docs/core/compatibility/toc

[tarekgh]

LGTM, Thanks!