Add information about .gitignore files

[NeuroShepherd]

1. I frequently see new users committing e.g. .DS_Store files and other unneeded content. We should include content about the global .gitignore and local .gitignore files, and recommend the use of usethis::git_vaccinate().
2. Also, some users need to ignore sensitive data. Recommend they create a data/sensitive directory that will be added to the .gitignore

[MalikaIhle]

I wonder whether this is just too much information for that first tutorial.
Perhaps this is better suited in the 'get more from git tutorial'?
with the branches, and gitignore? with indeed a process for sensitive data see https://the-turing-way.netlify.app/project-design/sdpw/pd-sdp-sensitive-files
we could still link to it at the mention you can ignore the gitingore file for now, but learn how and why to use it here.