docs: add diagram for event bus understanding

docs/develop/event-bus/index.md

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+# Understanding the event bus data flow
+
+For general information about the event bus, hava a look [here](../../../src/catalog/event-bus/README.md).
+
+The following diagram demonstrates the data flow on the event bus:
+
+![Event Bus Data flow](../../img/gs_cloud_eventbus_diagram.svg)
+
+1. Changes on the catalog/config level are usually done via the REST interface or the WebUI (via the Gateway)
+2. Changes are persisted in the catalog/config
+3. The `CatalogApplicationEventPublisher` listens to the events of the (native) GeoServer/Catalog (triggered by step 2)
+4. Whenever such an event fires, the `CatalogApplicationEventPublisher` will publish a "local" `GeoServerEvent`. Have a look [here](../../../src/catalog/events/README.md) for the full type hierarchy.
+5. The `RemoteGeoServerEventBridge` (listens to these `GeoServerEvent`s and) broadcasts `RemoteGeoServerEvent`s to the event bus.
+6. All registered microservices listen for incoming `RemoteGeoServerEvent`s
+7. The payload of these remote events will be published as local events to reload/refresh the catalog/config locally.