Add NSubstituteDefaultFactory configuration documentation

[Copilot]

Addresses #837 by adding comprehensive documentation for customizing NSubstitute's auto value generation using the NSubstituteDefaultFactory configuration pattern.

What this adds

This PR adds a new "Customizing auto values" section to the auto-and-recursive-mocks documentation that explains how to extend NSubstitute's behavior following the open/closed principle. The documentation was requested to make the valuable guidance from PR #705 discussion officially discoverable.

Key features documented

• Container customization: How to use NSubstituteDefaultFactory.DefaultContainer.Customize() to create a derived container
• Decoration pattern: Using .Decorate<T>() to enhance existing implementations without replacing them
• Module Initializers: Proper setup using [ModuleInitializer] for early configuration
• Complete working example: Custom Task auto value provider that returns failed tasks instead of default completed tasks

Example usage

[ModuleInitializer]
internal static void ConfigureNSubstitute()
{
    var customizedContainer = NSubstituteDefaultFactory.DefaultContainer.Customize();
    customizedContainer.Decorate<IAutoValueProvidersFactory>(
        (factory, _resolver) => new CustomAutoValueProvidersFactory(factory));

    SubstitutionContext.Current = customizedContainer.Resolve<ISubstitutionContext>();
}

This enables scenarios like:

• Unity's UniTask support instead of standard Task
• Custom async patterns and domain-specific value generation
• Enhanced testing scenarios with specific auto-generated values

The documentation includes usage examples, key architectural concepts, and maintains consistency with NSubstitute's existing documentation style.

Note: This is purely documentation - no code changes to NSubstitute itself were needed as the API already existed.

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