doc: Make search more prominent

[treeman]

Currently we have to click a small link under The standard library to be able to search, which I do a lot. It should be easier to find the search function.

Perhaps a direct link or a search box at the top of http://doc.rust-lang.org/ would be good.

[steveklabnik]

Closing as dup of #14572