Fix #396: Use `<link rel=prev/next>` for [ and ] commands

Fix #396: Use `<link rel=prev/next>` for [ and ] commands

Commit 52974ef says:

> We don’t bother with `<link>`s since the commands are substitutes for clicking
> a visible element. (And at least in my experience, they’re not commonly used.)
> Moreover, if there’s a `<link>` with a `rel` attribute, there’s likely a
> corresponding `<a>`, too.

But that's wrong. `<link rel=prev/next>` is actually a _better_ candidate for
the previous or next page than looking for a link to them.