docs(events): update docs about events-backend used with the new backend system

Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

docs/backend-system/building-backends/08-migrating.md

@@ -632,7 +632,7 @@ A basic installation of the events plugin looks as follows.
 ```ts title="packages/backend/src/index.ts"
 const backend = createBackend();
 /* highlight-add-next-line */
-backend.add(import('@backstage/plugin-events-backend'));
+backend.add(import('@backstage/plugin-events-backend/alpha'));
 ```
 
 If you have other customizations made to `plugins/events.ts`, such as adding
@@ -646,6 +646,7 @@ depends on the appropriate extension point and interacts with it.
 
 ```ts title="packages/backend/src/index.ts"
 /* highlight-add-start */
+import { eventsServiceRef } from '@backstage/plugin-events-node';
 import { eventsExtensionPoint } from '@backstage/plugin-events-node/alpha';
 import { createBackendModule } from '@backstage/backend-plugin-api';
 /* highlight-add-end */
@@ -663,7 +664,28 @@ const eventsModuleCustomExtensions = createBackendModule({
       async init({ events /* ..., other dependencies */ }) {
         // Here you have the opportunity to interact with the extension
         // point before the plugin itself gets instantiated
-        events.addSubscribers(new MySubscriber()); // just an example
+        events.addHttpPostIngress({
+          // ...
+        });
+      },
+    });
+  },
+});
+/* highlight-add-end */
+
+/* highlight-add-start */
+const otherPluginModuleCustomExtensions = createBackendModule({
+  pluginId: 'other-plugin', // name of the plugin that the module is targeting
+  moduleId: 'custom-extensions',
+  register(env) {
+    env.registerInit({
+      deps: {
+        events: eventsServiceRef,
+        // ... and other dependencies as needed
+      },
+      async init({ events /* ..., other dependencies */ }) {
+        // Here you have the opportunity to interact with the extension
+        // point before the plugin itself gets instantiated
       },
     });
   },
@@ -671,17 +693,11 @@ const eventsModuleCustomExtensions = createBackendModule({
 /* highlight-add-end */
 
 const backend = createBackend();
-backend.add(import('@backstage/plugin-events-backend'));
+backend.add(import('@backstage/plugin-events-backend/alpha'));
 /* highlight-add-next-line */
 backend.add(eventsModuleCustomExtensions());
-```
-
-This also requires that you have a dependency on the corresponding node package,
-if you didn't already have one.
-
-```bash
-# from the repository root
-yarn --cwd packages/backend add @backstage/plugin-events-node
+/* highlight-add-next-line */
+backend.add(otherPluginModuleCustomExtensions());
 ```
 
 Here we've placed the module directly in the backend index file just to get