Add admonition to clarify the sign of the gz in gravity inversion (#56)

Add an admonition in the gravity inversion tutorial to better explain
the expected sign of the gz component in the `Data`.

Author: santisoler

notebooks/03-gravity/inv_gravity_anomaly_3d.ipynb

@@ -383,6 +383,23 @@
     "The SimPEG [Data](xref:simpeg#simpeg.data.Data) class is required for inversion and connects the observed data, uncertainties and survey geometry."
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {
+    "execution": {
+     "iopub.execute_input": "2025-06-02T16:23:34.738628Z",
+     "iopub.status.busy": "2025-06-02T16:23:34.738355Z",
+     "iopub.status.idle": "2025-06-02T16:23:34.744628Z",
+     "shell.execute_reply": "2025-06-02T16:23:34.743489Z",
+     "shell.execute_reply.started": "2025-06-02T16:23:34.738604Z"
+    }
+   },
+   "source": [
+    ":::{important}\n",
+    "SimPEG uses a right-handed xyz coordinate system in which the z axis points _upward_. When defining the `Data` object for a gravity inversion we must check that the `dobs` array contains the **upward** component of the gravity acceleration due to anomalous bodies.\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 8,