Document YAML python.use_system_site_packages option

[humitos]

A user raised (#4362 (comment)) that it was hard to find this option

python:
   use_system_site_packages: true

I checked the docs and I didn't find anything about this: https://docs.readthedocs.io/en/latest/yaml-config.html

We need to extend our docs to mention this in the YAML config page.

[stsewd]

ha, I thought the user check the option from the web interface. Also, I was documenting this as a new option for the v2