Motivation
A Python-flavoured gitignore would be good to include. I'm a bit surprised we don't get this from uv. Some thought needed on the best contents for this - GitHub has a template but I'm not sure I like everything it includes.

The name is a bit annoying because usethis tool pyproject.toml and usethis tool requirements.txt imply we should use usethis tool .gitignore but I don't like a leading dot. I also wonder here whether this implies these should all live under a different command like usethis file.

Summary of desired enhancement
Consider an interface for adding a .gitignore file.