Added section to Python models doc to discuss third party packages following this the thread: https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1730272033637189

Added section to Python models doc to discuss third party packages following this the thread: https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1730272033637189 #25801