rustdoc: Larger click areas for sidebar items

There was feedback gathered by @bjz which request for larger click targets in the rustdoc sidebar. Here's my attempt at that.

My only concern with this patch is the removal of `<br>` between sidebar links. This may break formatting for text-only viewers of this documentation. If there is a large enough demographic of people that will be affected, perhaps we can try switching the structure of each sidebar block sidebar to an `<ol>` with each item as a `<li>`. 

* Change links to display:block for click larger targets
* Remove linebreaks due to extra space
* Adjust margins so that element spacing stays the same
* Sidebar item hover background colour chosen from `<pre>` styling