Allow moving focus to search with slash (`/`)

[parlough]

Describe the problem

Keyboard user's can't super easily navigate to search if their focus is elsewhere. Also many sites support jumping to search with /, but we don't, breaking potential user expectations.

Expected fix

Set up a / key binding to focus the search bar. If the sidenav is expanded on narrow devices, it should focus the search bar in there instead of the one at the top of the menu.

Additional context

Implemented on dart.dev in dart-lang/site-www#3844