message: Add link to the Rust Cookbook to the topbar
diff: </a>
  </li>
  - <li class="pure-menu-item menu-item-divided">
  + <li class="pure-menu-item">
  <a href="https://doc.rust-lang.org/rust-by-example/" target="_blank"
  class="pure-menu-link">
  Rust by Example
  </a>
  </li>
  + <li class="pure-menu-item menu-item-divided">
  + <a href="https://rust-lang-nursery.github.io/rust-cookbook/" target="_blank"
  + class="pure-menu-link">
  + Rust Cookbook
  + </a>
  + </li>
  +
  <li class="pure-menu-item">
  <a href="https://crates.io" target="_blank" class="pure-menu-link">
  Crates.io
  