What is this "dd" doc format?

[John Gabriele via Digitalmars-d]

I just went looking for the source for the dlang.org overview 
page, and found 
[this](https://github.com/dlang/dlang.org/blob/master/overview.dd).

I've seen and used a lot of markup formats, but have never run 
across this. What format is it? Also curious: for what reasons 
was it chosen over a more customary markup?

Thanks.