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.

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