docs: getting_started: change default python setup to use virtual env

Change default python setup suggested in getting started guide to use
virtual environments. This aligns with the documentation itself, which
suggests that users use virtual environments with Zephyr. The Windows
python setup is also updated with the correct command to create a
virtual environment, as "python3" is not a defined executable in a
windows commandline after installing python with chocolatey

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>