content/jupyter: Use timeit as first cell magic example

[rkdarst]

• Related: #64 (makes it slightly bettter but not fully fixing)
• Introduce %%timeit as first magic since %%bash doesn't work on all OSs.
• Not yet removing %%bash, I'd like to add another magic as an extra example to not make it just about %%timeit
• I think that %%prun is most likely option for another demo but it should have a longer example that gives a meaningful profile.