doc: Make search more prominent

Currently we have to click a small link under [The standard library](http://doc.rust-lang.org/#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.
