Fix firststeps_handout.qmd

[uekerman]

Things, I did not yet fix:

• [x] http://www.s-ccs.de/summerschool_simtech_2023/material/1_mon/firststeps/vscode.qmd does not yet exist
• [x] small “play” button on the top right -> no button for me, but one on the left (Run and Debug)
• [x] This is quite similar to %autoreload 2 in python -> only Jupyter, right?
• [x] What does "methods" do exactly? What is a "method"?
• [x] "inplace / side-effects endwith!()" is not self-explanatory enough