docs(system-design): add messaging and stream processing guide

Co-authored-by: Codex <codex@openai.com>

Author: shalshcode08

manifest.json

@@ -140,6 +140,10 @@
         {
           "title": "Resilience, Retries, Backpressure, and Overload Control",
           "path": "systemdesign/resilience-retries-backpressure-overload"
+        },
+        {
+          "title": "Messaging, Event-Driven Architecture, and Stream Processing",
+          "path": "systemdesign/messaging-event-driven-stream-processing"
         }
       ]
     }