redirect contrib/apidoc/cargo/ to new hosting location of cargo api docs

Author: jofas

src/doc/contrib/book.toml

@@ -4,3 +4,6 @@ authors = ["Eric Huss"]
 
 [output.html]
 git-repository-url = "https://github.com/rust-lang/cargo/tree/master/src/doc/contrib/src"
+
+[output.html.redirect]
+"/apidoc/cargo/" = "https://doc.rust-lang.org/nightly/nightly-rustc/cargo/"