[Xfce-i18n] Add "overflow-y: scroll;" to website CSS code

[scootergrisen]

I suggest adding:

     html {
         overflow-y: scroll;
     }

To the different pages on http://www.xfce.org/ like:
http://docs.xfce.org/
http://archive.xfce.org/

So the content does not jump left/right depending on how much content 
there are on the page.