better explain vectorisation

[colinsauze]

make it clear that vectorisation lets us apply an array to a scalar function

[colinsauze]

add a diagram like this one: https://ars.els-cdn.com/content/image/3-s2.0-B9780128038192000069-f22-01-9780128038192.jpg
or this one: https://miro.medium.com/v2/resize:fit:720/format:webp/1*1CReEwIob8gL4Kuouz7yyA.png