reintroduce manual link in doc index

whoops

Fixes https://github.com/rust-lang/rust/pull/16827#issuecomment-55160273
auto merge of #17153 : steveklabnik/rust/add_link_to_manual, r=sfackler

whoops

Fixes https://github.com/rust-lang/rust/pull/16827#issuecomment-55160273