Write more/adjust guidance re: figures

[yatil]

https://www.scottohara.me/blog/2019/01/21/how-do-you-figure.html

We use figure with role=group once, that should change to figure. Apart from that, I think we're ok but it might be good to add a sentence or two that says “Figcaptions are not alt text” (and link to above article.)