[docs] Improvements to the sys.path initialization documentation

Hi All, Steve Dower, Eryk Sun and I have been working on improving the documentation around how sys.path is initialised and therefore how Python modules are found. We're moving the details from https://docs.python.org/3.11/using/windows.html#finding-modules to a platform-agnostic new section in https://docs.python.org/3.11/using/cmdline.html We're aiming to have one place in the documentation where all the ways to customize sys.path (PYTHONPATH, ._pth, site, etc) are at least mentioned. We'd appreciate further community feedback on the changes before they are merged. https://bugs.python.org/issue31582 https://github.com/python/cpython/pull/31082 Thank you, Russel