Update docs as per Luca's suggestions

docs/index.rst

docs/quick_start/QuickStart.rst

docs/quick_start/code_ref.md

docs/tutorial/0_intro.md

docs/tutorial/1_command_line.md

docs/tutorial/3_classification.md

docs/tutorial/command_line/cv/3d_model.png

src/python/postprocess/plot/parity_plot.py

src/python/postprocess/plot/utils.py

src/python/py_binding_cpp_def/bindings_docstring_keyed.cpp

src/utils/string_utils.cpp

src/utils/string_utils.hpp