doc: getting_started: simplify Python dependency install step

The original wording made the `west packages` step look optional, which
caused confusion for users. Rephrase it as a direct instruction to make it
clear this step is required.

ref: https://github.com/zephyrproject-rtos/zephyr/discussions/94572



Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>