feat(chapter-1): understanding commits in Git

Revamps chapter one "Understanding Commits in Git". Pinging @cburgdorf for review.

Comments welcome!
- as a guide for people **who are** already familiar (the wording wasn't incorrect, but I think this is better and more clear)
- second sentence is a bit shady. Though I do get what you mean, I would phrase it differently, especially the part about *leaving out many more basic features of Git*. You could use **miss out on many basic features of Git** for instance, but even that might not be too clear. Maybe merge the second and third sentence, since they both kind of point to the same thing