Add GridSearchCV + train-test split figure in first GridSearchCV notebook

[lesteve]

It would be nice to have the diagram in this notebook close to the code creating the GridSearchCV object https://inria.github.io/scikit-learn-mooc/python_scripts/parameter_tuning_grid_search.html

We have the figure already https://github.com/INRIA/scikit-learn-mooc/blob/main/figures/cross_validation_train_test_diagram.png

but for now it is only used in the nested cross-validation notebook https://inria.github.io/scikit-learn-mooc/python_scripts/parameter_tuning_nested.html#with-hyperparameter-tuning