Added min-height to article of document

Current document is broken layout when article is short, as follows:

* http://www.playframework.com/documentation/2.2.x/SBTSettings
* http://www.playframework.com/documentation/2.3-SNAPSHOT/ThirdPartyTools
* http://www.playframework.com/documentation/2.3-SNAPSHOT/Issues

css styles don't consider the sidebar height, and so I fixed it.

But I have a problem.
It is that original css styles looks harmless on local server, because local layout is different from official web layout.
So I can't check absolutely...

I've upload screenshots to make sure.

## before

* official web

![web](http://gyazo.com/033a4bc9957825d06ec9896eda8a126d.png)

* local

![local](http://gyazo.com/0d1b04e7e5baa22f1e8570b7ac711ae1.png)

## after

* official web

![web1](http://gyazo.com/65d1a08bd9210a0952734d546c1be922.png)

![web2](http://gyazo.com/b7cc9830c548393ce640d38d611575ca.png)

* local

![loca](http://gyazo.com/2da3247dbca3c808eae26d0de9bcb84d.png)