Clarify size() of screenshots / canvas

[shiffman]

From @alignedleft nature-of-code/The-Nature-of-Code-archive#351

Screenshots / interactive canvas are 800x200 for book design, but live as 640x360 in github repo. This should be clarified as per nature-of-code/The-Nature-of-Code-archive#351.

I'm not going to do this yet in case the design changes. I should add the note here with the first example:

https://github.com/shiffman/The-Nature-of-Code/blob/master/chapters/00_7_intro.html#L178

[shiffman]

800x200 was always an awkward set of dimensions for the code examples and screenshots. Hopefully with the new p5 second edition, ideally we'll tweak the design of the book to have a more standard 16:9, 4:3, or square aspect ration.

[shiffman]

Hi @jasongao97 here is another important one to consider related to the build of the web version of the book!

[shiffman]

This has been resolved with nature-of-code/noc-book-2#90