Redirect doc.crates.io to doc.rust-lang.org/cargo

[behnam]

This is the last step of the migration of the Cargo docs.

Background: rust-lang/rust#44910

We need to wait until rust-lang/rust#45692 makes it to stable,
so https://doc.rust-lang.org/cargo/ won't redirect to http://doc.crates.io/ anymore, and then we can set up the redirect.

What needs to be done

Use MIGRATION_MAP to create redirect from old doc.crates.io URLs to new doc.rust-lang.org/cargo ones, all with permanent status.

[alexcrichton]

Done!