Reference to a package main page

[Julow]

It's not possible to reference a package main page, which is not a special concept in Odoc but currently a regular page (https://github.com/ocaml/odoc/issues/1037).

Drivers currently change the name of the index.mld page and there's no conventional name (Dune names it <package name>.odoc but Voodoo names it <version string>.odoc).

This might also be used to write unambiguous fully-qualified references in the future (eg. package_name.library.Module).