Fix horizontal scrolling / width on mobile

[andys8]

The video shows that the start page haskell.org behaves glitchy on mobile. You can scroll horizontally, but you shouldn't be able to. This doesn't result in a terrible user experience, but still is an issue that should be avoided.

Record_2023-02-03-12-20-47.mp4

I tested with Android 13 and latest Chrome and Kiwi Browser on a OnePlus 8.