new directives to complement 'challenge'

[rkdarst]

From @bast: I would find "type along" "try on your own" and "watch as demo" more useful than "challenge".

I agree. I would add exercise, demo and type-along - the main reason I haven't is that current directives are based on CR jekyll-common (and thus I guess carpentries?) - do we want to maintain semantic compatibility with that, or has that time sort of gone past?

(perhaps they had a reason for their choices, too, which we should consider)