Add a "Concurrency" section covering child_process, worker_threads, and cluster

[omscse]

Node.js ships three built-in concurrency mechanisms — child_process, worker_threads, and cluster — but the Learn site doesn't currently have a section that explains how they relate to each other or when to reach for which one. Each is documented individually in the API reference, but there's no conceptual/comparison content here.

Proposing a new Concurrency section with an initial article:

• Comparing Node.js concurrency models — overview of the event loop's single-threaded model, a side-by-side comparison table, worked examples for each of the three APIs, and a decision guide for choosing between them.

I have a draft ready and will open a PR shortly implementing this.

[avivkeller]

No need to open an issue immediately followed by a PR