404 page assets need absolute, not relative links.

[zkamvar]

What I did not realize is that the GitHub server will serve a 404 from the URL that generated the 404. This means that any relative links will be relative to the incorrect page. This is a problem in our most needed cases like https://carpentries.github.io/instructor-training/checkout/index.html

[zkamvar]

The solution was to add list(site = list(root = url)) to the 404 page metadata to modify this part of the base data for pkgdown