docs: add poppler-utils requirement to Step 11 backend setup

- Add system dependency note for poppler-utils in Step 11
- Required for PDF document processing/extraction
- Include installation instructions for Ubuntu/Debian, macOS, Windows
- Note that Docker Compose already includes it
- Helps users avoid document processing failures

Fixes: Document extraction failing due to missing poppler-utils

Author: T-DevH

notebooks/setup/complete_setup_guide.ipynb

@@ -3183,7 +3183,30 @@
     "# Backend API: http://localhost:3000/api/\n",
     "```\n",
     "\n",
-    "**Note:** The notebook below uses Option 1 (direct start) for development. For Option 2, use Docker Compose commands.\n"
+    "**Note:** The notebook below uses Option 1 (direct start) for development. For Option 2, use Docker Compose commands.\n",
+    "\n",
+    "### ⚠️ System Dependencies for Document Processing\n",
+    "\n",
+    "**Important:** If you plan to use document extraction features (PDF processing), you need to install `poppler-utils` on your system:\n",
+    "\n",
+    "**Ubuntu/Debian:**\n",
+    "```bash\n",
+    "sudo apt-get update\n",
+    "sudo apt-get install poppler-utils\n",
+    "```\n",
+    "\n",
+    "**macOS:**\n",
+    "```bash\n",
+    "brew install poppler\n",
+    "```\n",
+    "\n",
+    "**Windows:**\n",
+    "- Download from: http://blog.alivate.com.au/poppler-windows/\n",
+    "- Or use: `choco install poppler`\n",
+    "\n",
+    "**Note:** If you're using Docker Compose (Option 2), `poppler-utils` is already included in the backend container. This requirement only applies when running the backend directly (Option 1).\n",
+    "\n",
+    "After installation, restart the backend server for the changes to take effect.\n"
    ]
   },
   {