Add exercises

[tompollard]

Raised in carpentries-lab/reviews#23 (comment):

Opportunities for formative assessments are included and distributed throughout the lesson sufficiently to track learner progress. (We aim for at least one formative assessment every 10-15 minutes.)

Episodes 5 and onwards don't have any exercises listed on the episode page but they do show time for exercises at the top of the page. Maybe these are in Python and not written up?