docs(postgresql): add indexing guide (#5)

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

Author: shalshcode08

manifest.json

@@ -21,7 +21,8 @@
         {
           "title": "Schema Design and Data Modeling",
           "path": "postgresql/schema-design-data-modeling"
-        }
+        },
+        { "title": "Indexing", "path": "postgresql/indexing" }
       ]
     }
   ]