Add comprehensive annotations documentation for .NET Aspire

[Copilot]

This PR adds comprehensive documentation for annotations in .NET Aspire, addressing a significant gap in the documentation where annotations are used extensively across the stack but lacked centralized coverage.

What's Added

📄 New Documentation: docs/fundamentals/annotations-overview.md

• What are annotations: Complete explanation of the annotation system and IResourceAnnotation interface
• Built-in annotation types: Coverage of key annotations like EndpointAnnotation, ResourceUrlAnnotation, EnvironmentCallbackAnnotation, ContainerMountAnnotation, and CommandAnnotation
• Creating custom annotations: Step-by-step guide with working examples
• Advanced patterns: Conditional annotations, composition, validation, and testing approaches
• Best practices: Guidelines for naming, documentation, and maintaining focused responsibilities

💻 Working Code Examples: docs/fundamentals/snippets/annotations-overview/

All code examples are fully compilable and demonstrate:

• Custom annotation implementation patterns
• Extension method patterns following .NET Aspire conventions
• Annotation processing and inspection techniques

🗂️ Navigation Integration

• Added to Fundamentals section in docs/toc.yml with appropriate search keywords

Key Features Addressed

✅ General usage documentation: Comprehensive coverage of how annotations work across the dashboard, deployment tools, testing infrastructure, and hosting layer

✅ Custom annotation creation: Complete implementation guide showing how to:

// 1. Define custom annotation
public sealed class CustomConfigAnnotation : IResourceAnnotation
{
    public string ConfigKey { get; set; } = "";
    public string ConfigValue { get; set; } = "";
}

// 2. Create extension methods
public static IResourceBuilder<T> WithCustomConfig<T>(
    this IResourceBuilder<T> builder, string key, string value)
    where T : class, IResource
{
    return builder.WithAnnotation(new CustomConfigAnnotation 
    { 
        ConfigKey = key, 
        ConfigValue = value 
    });
}

// 3. Use in app host
var api = builder.AddProject<Projects.Api>("api")
    .WithCustomConfig("feature-flag", "enabled");

Quality Assurance

• ✅ All markdown linting passes
• ✅ All code examples compile successfully
• ✅ Follows Microsoft Writing Style Guide conventions
• ✅ Proper API cross-references using <xref> syntax
• ✅ Consistent with existing .NET Aspire documentation patterns

This documentation provides developers with the knowledge needed to effectively use and extend .NET Aspire's annotation system, filling a critical gap in the current documentation set.

Fixes #1614.

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/annotations-overview.md	Resource annotations in .NET Aspire
docs/toc.yml	docs/toc

[davidfowl]

This is not exhuastive. @IEvangelist is it supposed to be?