Updating documentation links for exercises i_2, i_3, and i_4

[swozniak]

Hi! I started to go through these exercises, but saw room for clarity in the links to documentation for the first few exercises. There's plenty to learn, and I wouldn't want to see others confused by the convention for documentation references so early on. Submitting a PR in case this feels suitable to your team as well. Thank you for this resource overall!