New DUB documentation

[BoQsc]

Also it's a good idea to maintain the same style as dlang forum 
and dlang website, along with dlang tour and online dlang editor. 
The dlang webdesign choice feels really solid and robust. At 
least from my perspective.

If someone needed a dark theme dub documentation. A simple button 
to toggle it would have been enough.

Small tweaks for layout and small tweaks overall for more 
professional look and efficiency of use. Would have been enough.