docs(postgresql): add replication and scaling guide (#11)

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

Author: shalshcode08

manifest.json

@@ -38,6 +38,10 @@
         {
           "title": "Performance Tuning and Operations",
           "path": "postgresql/performance-tuning-operations"
+        },
+        {
+          "title": "Replication and Scaling",
+          "path": "postgresql/replication-scaling"
         }
       ]
     }