docs(system-design): add observability and SRE guide (#46)

* docs(system-design): add observability and SRE guide

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

* chore(system-design): format observability guide

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

---------

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

Author: shalshcode08

manifest.json

@@ -148,6 +148,10 @@
         {
           "title": "Messaging, Event-Driven Architecture, and Stream Processing",
           "path": "systemdesign/messaging-event-driven-stream-processing"
+        },
+        {
+          "title": "Observability, SRE, Capacity Planning, and Incident Response",
+          "path": "systemdesign/observability-sre-capacity-planning-incident-response"
         }
       ]
     }