[v3] Write a brief benchmarking guide

A lot of students are interested in finding out how fast their solution runs, how they can optimize it etc. Often they use `@time` instead of `@btime` and then run into compliation times, or end up with meaningless results because the compiler treats everything as a constant.

The planned performance exercise (#353) will go into more detail but it would be good to have a very brief guide on how to benchmark one's code as part of the track docs.