Add extensibility article on how to write non-container custom resources

[Copilot]

This PR adds a new documentation article that explains how to create custom .NET Aspire resources that don't rely on containers, addressing the need for guidance on non-container extensibility patterns.

What's Added

• New article: docs/extensibility/custom-non-container-resource.md - Comprehensive guide on creating non-container custom resources
• Working code example: HTTP proxy resource demonstrating lifecycle hooks and dashboard integration
• Supporting snippets: Complete, buildable code samples in docs/extensibility/snippets/HttpProxyResource/
• Updated navigation: Added the new article to the table of contents and cross-referenced from existing articles

Key Content

The article covers:

1. When to use non-container resources vs simpler approaches like AddConnectionString() or AddParameter()
2. Core implementation patterns using IResource, IResourceWithEndpoints, and extension methods
3. Lifecycle management with IDistributedApplicationLifecycleHook for startup/shutdown coordination
4. Dashboard integration through standard .NET logging and notes on advanced notification services
5. Practical example with an in-process HTTP proxy that demonstrates all concepts

Example Usage

var builder = DistributedApplication.CreateBuilder(args);

// Add a non-container HTTP proxy resource
var proxy = builder.AddHttpProxy("api-proxy", "https://jsonplaceholder.typicode.com");

var webapp = builder.AddProject<Projects.WebApp>("webapp")
                    .WithReference(proxy);

builder.Build().Run();

The implementation shows how to:

• Hook into application lifecycle events
• Manage background services and cleanup
• Integrate with the Aspire dashboard through logging
• Provide meaningful status updates to developers

Fixes #792.

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Internal previews

📄 File	🔗 Preview link
docs/extensibility/custom-hosting-integration.md	docs/extensibility/custom-hosting-integration
docs/extensibility/custom-non-container-resource.md	docs/extensibility/custom-non-container-resource
docs/toc.yml	docs/toc