Auto merge of #4461 - behnam:policies, r=alexcrichton

[doc] Redirect /policies.html to crates.io/policies

This relieves us from migrating the Package Policies document into the
new Cargo Manual mdbook.

Moved the content to `crates.io` repo in
<https://github.com/rust-lang/crates.io/pull/1033>.

NOTE: We should wait until that change goes live before landing this
one. I'll notify here when it's time to land.

Closes <https://github.com/rust-lang/crates.io/issues/1030>