14652 frontpage style

The issues addressed are mozilla/rust#14651 and mozilla/rust#14652.

Relative to 14651 (coding style of the frontpage), the println! line spacing was off.  In addition, I updated the comments to make them more clear, since the current version doesn't seem intuitive to a new reader. 

Relative to 14652, I added a button link to the playground. This repository seems to be the source for the main website; is the source for the playground also available on Github? Ideally, linking to the playground would copy the current code in the editor and paste it into the playground, but I'm not exactly sure how to do this without looking at the playground source.  I would also like to implement different programs (such as Hello World) with a selector on the main page so additional code examples can be seen. 
Mixing in HTML with JS isn't good style. Instead, place the HTML with the rest of the markup and simply change the HREF when the result is changed. Note: What do you do when there is an error, or when the code in the editor has changed but hasn't been run?