Experiment with shorter line width for body copy, and larger paragraph margins

[Rich-Harris]

our line width is a bit on the large side, we could maybe nudge it down a smidge

https://x.com/hovhaDovah/status/1848814886534459719

[AleksandrHovhannisyan]

Forgot to mention this, but iirc I used a line width of ~640px in my screenshot (for reference). That was just a first pass, though; it doesn't have to be that tight.

[Rich-Harris]

closed via #632