Add comprehensive documentation for ExecutableResource and AddExecutable API

[Copilot]

This PR adds a new comprehensive article documenting how to use ExecutableResource and the AddExecutable extension method in .NET Aspire app hosts. The documentation was missing entirely, leaving developers without guidance on how to host external executable applications.

What's included

The new article docs/app-host/executable-resources.md covers:

• Basic usage of AddExecutable with command-line arguments and working directories
• Command-line argument configuration using multiple approaches (WithArgs, direct parameters)
• Resource dependency management with WithResource calls and environment variable access
• Practical Vercel CLI example demonstrating real-world usage
• Publishing scenarios with PublishAsDockerfile for deployment
• Advanced scenarios including conditional execution and complex argument chains
• Best practices for production use

Key examples

The article includes practical examples for common scenarios:

// Basic executable with arguments
var app = builder.AddExecutable("vercel-dev", "vercel", "dev", ".")
    .WithArgs("--listen", "3000");

// Resource dependencies
var redis = builder.AddRedis("cache");
var app = builder.AddExecutable("worker", "python", "worker.py", ".")
    .WithReference(redis)      // Provides connection strings
    .WithEnvironment(context => {
        context.EnvironmentVariables["REDIS_HOST"] = 
            redis.Resource.PrimaryEndpoint.Property(EndpointProperty.Host);
    });

// Publishing for deployment
var app = builder.AddExecutable("frontend", "npm", ".", "start")
    .PublishAsDockerfile();

The documentation addresses the specific use case mentioned in the issue (external executable hosting) while providing comprehensive coverage for any external executable scenario using modern tooling like Vercel CLI.

Fixes #1381.

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Internal previews

📄 File	🔗 Preview link
docs/app-host/executable-resources.md	docs/app-host/executable-resources
docs/toc.yml	docs/toc

[IEvangelist]

Why do you always mistakenly not account for the .github/copilot-instructions.md file? You're supposed to use complete sentences and proper punctuation for lists.

[Copilot]

Fixed the list formatting by adding proper punctuation (periods) to complete sentences as specified in the .github/copilot-instructions.md file.

[IEvangelist]

Was this done for the entire document, or just the single section I commented on. It needs to apply to all MD files you create.

[Copilot]

Applied the punctuation fix to the entire document. All lists containing complete sentences now have proper periods, including the method parameters section (lines 48-51) and the best practices section (lines 265-270). Commit e6c968e.