Don't show sidebar by default on small screens if javascript is disabled

[TLATER]

Hi, I usually browse the web without JavaScript, unless it's necessary - saves bandwidth, makes pages load faster and conserves a bit of battery life, while tripping up most trackers not trivially defeated by ad block.

This normally works well for documentation sites, but not for nix.dev because the sidebar is expanded over the site content:

In principle this is good, because it keeps the site navigatable on large screens, but it would be nice if this was expanded elsewhere (maybe top of the page) or not at all for small screens (i.e., anything with a screen width of less than the sidebar width + content width - this should be just about doable with CSS media things).