doc: gsg: Remove misleading mention of Powershell

While we will probably want to properly document how to use Powershell
on Windows in the near future, current Getting Started Guide contained a
misleading mention to activating Pythonv venv using Powershell that this
commit removes.

Addresses comments raised in #64682.

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