Added cookbook page for publish-subscribe pattern (#7)

Author: BlackBearFTW

src/routes/docs/[...6]cookbook/[...7]pub-sub/+page.md

@@ -0,0 +1,36 @@
+---
+title: Publish-Subscribe pattern
+description: Cookbook example showing how to implement the publish-subscribe pattern
+---
+
+# {$frontmatter.title}
+
+If you are coming from other popular mediator libraries, you may be looking for a way to implement the
+[publish-subscribe pattern](https://learn.microsoft.com/en-us/azure/architecture/patterns/publisher-subscriber)
+with Immediate.Handlers. A common use case is implementing domain events.
+
+Immediate.Handlers fully supports this pattern, but it intentionally does not provide a built-in `Publisher`
+abstraction. Maintaining an opinionated publish-subscribe implementation is out of scope and would limit
+the flexibility required by different projects.
+
+An example is provided here of a simple `Publisher` which works for many cases.
+
+```cs
+public sealed class Publisher<TNotification>(
+    IServiceProvider serviceProvider
+)
+{
+    public async ValueTask Publish(TNotification notification)
+    {
+        foreach (var handler in serviceProvider.GetServices<IHandler<TNotification, ValueTuple>>())
+        {
+            await handler.Handle(notification);
+        }
+    }
+}
+```
+
+Finally, this implementation can simply be registered as singleton service:
+```cs
+services.AddSingleton(typeof(Publisher<>));
+```