chapter 2 of tutorial (#945)

docs/index.rst

docs/tutorial.rst

docs/tutorial/a_minimal_working_example.rst

docs/tutorial/algorithm_interfaces.rst

docs/tutorial/customize_algorithms.rst

docs/tutorial/customize_environments_and_wrappers.rst

docs/tutorial/customize_training_pipeline.rst

docs/tutorial/images/alf_diagram.png

docs/tutorial/images/pipeline.png

docs/tutorial/summary_metrics_and_tensorboard.rst

...ial/understanding_ALF_via_the_minimal_working_example.rst