Lychee by itself does not have a way to explicitly declare the
intent for the site to be relocatable.
Recent-ish changes in lychee and/or its dependencies have made
it catch the a root reference in all mdbook 404.html pages,
which is quite confusing.
By improving the UX around this aspect of the site, we resolve
at least part of that confusion.
(Though mdbook users should probably exclude the 404 page)
The textual output format changed in the last update, so let's
switch this to JSON, so that it's less likely to be affected by
similar kinds of changes going forward.
