[BUG] Link from Cursive::new to CursiveExt doesn't work offline.

I'm not sure how to resolve this. I think the ease of getting and using documentation offline is one of the strength of Rust, but because `cursive_core::Cursive::new` is linking *downstream* it can't use "local" links, even though it's linking to a sibling crate, and will usually be generated as part of the `cursive` documentation & re-export.

Somewhat linked to rust-lang/rust#24305 "Pain points of reexports"
*Definitely* linked to rust-lang/rust#74481 "Figure out a way to link to downstream crates"