add docs for event emitter

Author: chargome

docs/platforms/javascript/guides/nestjs/features/event-emitter.mdx

@@ -0,0 +1,18 @@
+---
+title: NestJS Event Emitter
+description: "Learn about instrumenting NestJS event based services."
+---
+
+<Note>
+  The @nestjs/event-emitter module is auto-instrumented from version `@sentry/nestjs` 8.39.0 and up.
+</Note>
+
+The NestJS SDK wraps the `@OnEvent` decorator automatically to:
+
+- Create performance traces for event handler executions
+- Automatically capture any unhandled exceptions that occur in event handlers
+- Maintain visibility into asynchronous event-driven flows
+
+When an event handler is executed, a new span is created to track its performance, and any errors are automatically reported to Sentry while preserving the original error behavior.
+
+This instrumentation works transparently with existing NestJS event handlers without requiring any code changes to your application.