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 by passing arguments directly to the AddExecutable method
• Resource dependency management with WithReference calls and automatic connection string injection
• Practical Vercel CLI example demonstrating real-world usage
• Publishing scenarios with PublishAsDockerfile for deployment
• Best practices for production use

Key examples

The article includes practical examples for common scenarios:

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

// Resource dependencies with automatic connection string injection
var redis = builder.AddRedis("cache");
var app = builder.AddExecutable("worker", "python", ".", "worker.py")
    .WithReference(redis);      // Automatically provides connection strings

// 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.

Important: All examples correctly use the AddExecutable API signature where arguments are passed as params string[] args directly to the method. There is no WithArgs() method available on ExecutableResource.

Fixes #1381.

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