doc: getting_started: remove global Python setup

As it was already outlined, installing Python dependencies at a system
level or user level can be dangerous. For this reason, the default setup
instructions already pointed users to use Python virtual environments. On
newer Ubuntu installations (23.04 onwards) system-level pip installs are
not allowed. Let's make Zephyr _Python-safe_ as well by removing dangerous
instructions.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>