create exercise section

[lisa-wm]

@jakob-r, I just started to add an exercise section and realized that this is probably sth that should go directly into the theme, right? (if not, I'll do it myself)

• "Exercises" should appear at the top between "Chapters" and "Appendix"
• In the home section, "Exercises" should appear before "Cheat Sheets". If possible, could file URLs be listed in the box as with the chapters?
• The landing page should look as for cheat sheets (i.e., just a single .md to list the PDFs)
  Merci 🙏🏻

[jakob-r]

is solved