fix: Update gitignore with generated folder for Mondrian documentation

Author: Valentin-Laurent

.gitignore

@@ -14,6 +14,7 @@ doc/examples_classification/
 doc/examples_regression/
 doc/examples_calibration/
 doc/examples_multilabel_classification/
+doc/examples_mondrian/
 doc/auto_examples/
 doc/modules/generated/
 doc/datasets/generated/