If a `_index.md` file has `build.render = "never"` set, it ends up as an
ancestor but without a link. This means we render a link with the title
of the page, but with a blank link, which makes a link to the current
page. This is never useful.

e.g. on https://piscine.codeyourfuture.io/sprints/1/day-plan/ there's a
"Piscine" link that has an empty `href` (so points at the current page)
which shouldn't be there. This is because
`org-cyf-piscine/content/sprints/_index.md` sets
`build.render = "never"` because it's a single-module course, so the
extra level of nesting isn't so useful.