message: [doc/header] Update Policies URL
diff: <li><a href='environment-variables.html'>Environment Variables</a></li>
  <li><a href='source-replacement.html'>Source Replacement</a></li>
  <li><a href='external-tools.html'>External Tools</a></li>
  - <li><a href='policies.html'>Policies</a></li>
  + <li><a href='https://crates.io/policies'>Policies</a></li>
  </ul>
  </div>
  </div>
  