Add a FAQ entry about how to turn off `.venv` usage

[brettcannon]

Brought up by #261 . It's (hopefully) made clear by https://python-launcher.app/#determining-the-selected-interpreter , but it may not be obvious either.

[cas--]

I was left a bit confused with different Python version when I was running py in an old project directory with .venv but was not active. The confusion was compounded by switching pyenv versions...

I did find that documentation about printing out the interpreter path to diagnose the issue, although took an age to recall this project name 😄

An FAQ entry might be useful for clarity 🤔