DOC: Cleanup for nbsphinx output

Followup to #15581

Using the `nbsphinx: hidden` metadata to hide the ouptut, so
readers don't see matplotlib's fc-list warning.

Make the tables monospaced in CSS.

doc/source/style.ipynb

@@ -45,6 +45,20 @@
     "Let's see some examples."
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true,
+    "nbsphinx": "hidden"
+   },
+   "outputs": [],
+   "source": [
+    "import matplotlib\n",
+    "# We have this here to trigger matplotlib's font cache stuff.\n",
+    "# This cell is hidden from the output"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,

doc/source/themes/nature_with_gtoc/static/nature.css_t

@@ -330,6 +330,11 @@ tbody tr:nth-child(odd) {
     background: #f5f5f5;
 }
 
+table .data, table .row_heading table .col_heading {
+    font-family: monospace;
+}
+
+
 /**
  * See also
  */