Add diagrams to Arrows documentation

This adds diagrams to the documentation of Arrows, similar to the ones found on
https://www.haskell.org/arrows/.

It does not add diagrams for ArrowChoice for the time being, mainly because it's
not clear to me how to visually distinguish them from the ones for Arrow. Ideally,
you might want to do something like highlight the arrows belonging to the same
tuple or same Either in common colors, but that's not really possible with unicode.