Explain clearly that python / pip / other commands are not Python code

[merwok]

https://github.com/pypa/packaging-problems/issues/369 is the latest instance of a person trying to type python setup.py sdist or pip install something in the Python shell rather than in a system console, which is not obvious to everyone.

None of https://packaging.python.org/tutorials/packaging-projects and https://packaging.python.org/guides/distributing-packages-using-setuptools/#wheels say a word about that.