When a package version doesn't exist, still show the package 404 page that has the version switcher

[sabine]

When you go to https://ocaml.org/p/cmdliner/1.1.0/doc/Cmdliner/Cmd/index.html, you see the regular 404 page of OCaml.org.

However, it would be better to show the package 404 page that includes the list of available versions to switch between (Ocamlorg_frontend.package_documentation_not_found) - this helps people more gracefully recover from following backlinks to packages that were removed from opam for some reason.