Number the exercises

[dsmits]

• chapter 1
• chapter 2
• ch 3
• ch 4
• ch 5
• ch 6
• ch 7
• ch 8

[frankcorneliusmartin]

Number challenges only in the carpentries. (could also be after the workshop)

[CunliangGeng]

ch4 and ch6 have been numbered exercises before, so I removed myself from this issue.