Prefix absolute URLs on 404 page with the site base path (aka context path)
Deployed Antora sites don't always sit at the root of the domain. For example, organization pages on GitHub Enterprise have a base path of /<orgname>/.
For sites such as GitHub Pages which don't support server-side URL rewriting, root-relative paths generated by Antora must be prefixed with the site's base path or they won't work.
The concrete case where I'm seeing this is resources referenced from Antora's 404 page, none of which are found. For example, href="/_/css/site.css" should be href="/<orgname>/_/css/site.css".
Please support prefixing of root-relative paths generated by Antora with a configurable base path.
Related thought: Should Antora always generate root-relative paths instead of relative paths, e.g., for clarity?
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