Newly discovered Astro quirk: pages can be browsed with or without a trailing slash (or with /index.html), but relative links were breaking with the trailing slash since they weren't taking that extra directory layer into account (but only on the live site for some reason; they worked fine when running it locally).

It appears that if we instead link to a source .md or .mdx file, Astro instead generates an absolute link that will always work. This also has the advantage of making the links work on GitHub (which is why we don't just put in absolute links ourselves).