doc: getting_started: add Python virtual environment option
doc: getting_started: add Python virtual environment option

Python is used by the west meta-tool as well as by many scripts invoked
by the build system. It is easy to run into package incompatibilities
when installing dependencies at a system or user level. This situation
can happen, for example, if working on multiple Zephyr versions at the
same time. For this reason, the getting started guide has been updated
to offer instructions for both, using Python virtual environments and
installing globally. Python virtual environments has been added as
another choice for the reasons just mentioned.

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr