doc: develop: gsg: consolidate python3 install instructions for ubuntu

- Installing python3-venv should be done right when all dependencies are
  installed.
- Installing python3-venv is enough to pull wheel, setuptools
  (python3-setuptools-whl), and pip (python3-pip-whl) so that they are
  available in the venv, so reduce the list of packages to install
  accordingly.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>