Show use of `timeit` during Manual Profiling section

[LouiseABowler]

timeit is briefly mentioned in the Manual Profiling section, but no example of its use is given there. It is used frequently later on in the Optimisation section (e.g. Built in Functions, List Comprehension), so some introduction earlier on would be useful.

When we ran the course at King's in February, I added a short example to the Manual Profiling section, extending the comment that about timeit that was already present. That seemed to work well, so I'll write it up as a PR.