Docs: some links for inlined re-exports are broken

[robertbastian]

E.g. icu::list has broken ~crate-internal~ ([Part]) and crate-external ([Writeable]) links. The latter is an easy fix, we need to add a writeable dev-dependency, the former I need to look into.

[robertbastian]

Actually, Parts is also a crate-external link, it's writeable::Part. Adding the dev-dependency to the meta-crate fixes the Part and Writeable links, but not the writeable link to the crate.