Finishes first pass at converting intro to git slides to CLI

A few notes:

1. Slide 14 ("Tool Check" slide) needs instructions on how to open
the terminal in Windows.
2. Maybe some things could be added, like a section on `git rebase`
and/or going more into `git reset`. `git checkout` is useful on an
individual file basis as well. Even a small lecture about not
pushing to a shared branch unless you're sure.