* Add D3.js icon SVG
* Add D3.js icon details
Linked to the D3 official organisations repository for their logo. The
color is somewhat ambigious as their coloured logo uses gradients, and
there are no guidelines for the color (at least, I couldn't find them).
Hence, I went with one of the colors suggested by @SumNeuron in #874.
* Update D3.js icon details
