Move documentation to doc.crates.io

The actual crates.io domain will become the registry itself, but the
auto-generated documentation from this repository will continue to be available
at the doc.crates.io domain.

In the meantime, we've set up redirects from crates.io and www.crates.io to
doc.crates.io and the github-pages site will now be doc.crates.io
auto merge of #703 : alexcrichton/cargo/doc.crates.io, r=alexcrichton

The actual crates.io domain will become the registry itself, but the
auto-generated documentation from this repository will continue to be available
at the doc.crates.io domain.

In the meantime, we've set up redirects from crates.io and www.crates.io to
doc.crates.io and the github-pages site will now be doc.crates.io