Prior to this commit we'd generate a page for each MDX document as that
avoids having to load MDX dynamically (which can be tricky) or through
static props (which causes issues with serialization).
However, Vercel (which hosts for us) has an upper limit on the number of
routes, and because of this extensive documentation, we blew passed it
with this approach.
This changes to a dynamic resolution of MDX content with an async import
in the useEffect hook. This should drastically reduce the number of
routes and make Vercel happy.
I didn't do much digging into the effects of this on SSR. If it turns
out it's causes issues, we'll deal with it at that time.
