doc: Python doc, don't use twitter/commons as example

Back when Pants' source code lived in twitter/commons, it made sense to use example code from there. Nowadays, not so much. Use Pants' own src/python/example instead.

Instead of having sample code in the doc, "include" from src/python/example; that's easier to keep correct.

Testing Done:
build-support/bin/publish_docs.sh

Reviewed at https://rbcommons.com/s/twitter/r/654/