Document the use of `.gitignore` when authoring Python policy packs

[justinvp]

On this page: https://www.pulumi.com/docs/insights/policy/policy-packs/authoring/

Document the use of .gitignore (next to PulumiPolicy.yaml) for Python policy packs to ignore files/dirs like *.pyc and venv/ to prevent these from being included in the published policy pack's .tgz to keep the size of the policy pack small.