WIP draft of sets concept

[colinleach]

There are several choices of exercise, with not much agreement among other tracks. For R, I went with ozans-playlist. Python uses cater-waiter, Gleam and Java have gotta-snatch-em-all.

[depial]

about.md looks great! Every time I thought of a potential question, it was taken care of shortly after.

The only potential formatting nit is that I'm not sure the and in the description of intersect needs italics, since no italics are used elsewhere in similar circumstances (or maybe more italics could be used elsewhere?). Either way, definitely not an issue.

As for the exercises, I think I would lean towards cater-water, since it looks more comprehensive and encourages the use of a wider range of the functionality listed in about.md.