doc: update getting started pip3 --user info
doc: update getting started pip3 --user info

Using ``pip3 install --user`` installs Python packages in the Python
user install directory (``.local``) so we should remind folks to add
``~/.local/bin`` to their PATH.

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

punitvara/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr