Use sans-serif fonts in rustdoc

Rustdoc was previously using a serif font, Source Serif Pro. Sans-serif fonts are generally considered better for the web, so this changes rustdoc to use Source Sans Pro instead.

The reason I have made this change is that sans-serif fonts are *significantly* more common on the web than serif ones. Looking at Python’s docs, MDN, Java’s docs, and practically any random website I come across, I see nothing but sans-serif fonts for the body text. [Many](https://www.fonts.com/content/learning/fontology/level-2/making-type-choices/serif-v-sans-for-text) [websites](http://www.webdesignerdepot.com/2013/03/serif-vs-sans-the-final-battle/) [seem](http://www.scribe.com.au/tip-w017.html) [to recommend](http://webdesign.about.com/od/fonts/a/aa080204.htm) using sans-serif fonts for the web. (It should be noted that all of those sites themselves use sans fonts for the main body text!) Sans-serif fonts also give a more ‘modern’ feel, which certainly fits with the rest of rustdoc’s design and Rust in general.

![New rustdoc](https://i.imgur.com/l2wES5O.png)

This is a controversial change; any opinions would be appreciated.

cc #16173