Explain object oriented vs `plt.X` style to students

[klieret]

We should explain why using fig, ax = plt.subplots() and then ax.XXX is preferred by us over plt.XXX

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.