Make the line numbers of the source code clickable

While talking on IRC, someone wanted to post a link to the Rust source code, but while the lines of the rendered source code do have anchors (`<span id="[line number]">`), there is no convenient way to make links as they are not clickable. This PR makes them clickable.

Also, a minor fix of the FAQ is included.
I think this CSS rule was also used for clicking on headers in documentation. That may be covered elsewhere, but can you make sure that if you click on a header that it still gets highlighted?