docs(lite): explain the sidebar and its volatile files on the index page

docs/notebooks/index.ipynb

@@ -41,6 +41,22 @@
     "| [**Not your grandmother's toolbox**](./icra2021.ipynb) | The examples from the ICRA 2021 paper. |\n",
     "| [**Screw theory for robotics**](./iros2020.ipynb) | The IROS 2020 workshop tutorial. |\n"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "5f900b55",
+   "metadata": {},
+   "source": [
+    "## Working with files\n",
+    "\n",
+    "> **The sidebar.** The left-hand **sidebar** lists the local files, which are kept in a virtual file system within the browser (managed via IndexedDB).\n",
+    ">\n",
+    "> * To create a new notebook use *File/New/Notebook*\n",
+    "> * To upload a notebook or data file use the upload icon above the sidebar, or drag and drop files onto the sidebar\n",
+    "> * To load and open a file from a URL, use *File/Open From URL...*\n",
+    ">\n",
+    "> These files are volatile and vulnerable to browser cache clearing or session termination. You can download these files to your local drive by right-clicking a file in the sidebar and selecting *Download*.\n"
+   ]
   }
  ],
  "metadata": {