Implement a temp redirect for cargo docs

As discussed in
https://github.com/rust-lang/cargo/issues/4040#issuecomment-321639074

This is a redirect meant to be replaced once cargo docs have been
converted to mdbook. We just want *a* URL to ride the trains for now so
that we can print doc.rust-lang.org/cargo in the paper book and
guarantee that it will go *somewhere* useful by the time the book is
printed.

Implemented as a meta redirect in HTML because we don't currently have
any google juice at doc.rust-lang.org/cargo to lose.

src/bootstrap/builder.rs

@@ -257,7 +257,7 @@ impl<'a> Builder<'a> {
             Kind::Bench => describe!(check::Crate, check::CrateLibrustc),
             Kind::Doc => describe!(doc::UnstableBook, doc::UnstableBookGen, doc::TheBook,
                 doc::Standalone, doc::Std, doc::Test, doc::Rustc, doc::ErrorIndex, doc::Nomicon,
-                doc::Reference, doc::Rustdoc),
+                doc::Reference, doc::Rustdoc, doc::CargoBook),
             Kind::Dist => describe!(dist::Docs, dist::Mingw, dist::Rustc, dist::DebuggerScripts,
                 dist::Std, dist::Analysis, dist::Src, dist::PlainSourceTarball, dist::Cargo,
                 dist::Rls, dist::Extended, dist::HashSign),