add thumbnails

Author: SvenKlaassen

doc/examples/py_double_ml_cate.ipynb

@@ -371,7 +371,10 @@
     "collapsed": false,
     "pycharm": {
      "name": "#%%\n"
-    }
+    },
+    "tags": [
+        "nbsphinx-thumbnail"
+       ]
    },
    "outputs": [
     {