Crates sidebar menu in docs disappears when page width is less than 700px

[carols10cents]

This can be confusing; it makes it look like there aren't any docs available. Example:

@bruceadams was following along with Chapter 2 in the new book, and we mention cargo doc --open after adding the regex crate. At the time he ran the command, his browser window was under 700px wide because he had his screen split to have the book and editor/terminal open at the same time, so the docs opened themselves in that narrow window. The menu with the crates in the sidebars is just totally gone at that width, and that combined with the fact that the guessing game doesn't have any docs that would appear on the index page makes the docs look broken and like there aren't any docs to browse :(

Screenshot of this scenario:

The problem is this CSS:

@media (max-width: 700px) {
   // ...
  .sidebar .block {
    display: none;
  }
  // ...
}

It'd be awesome to have at least a hamburger menu or something to be able to get to the menu of crates even at narrower widths.