Add external links to the "Developer Documentation" page

Live preview: http://bitcoindevtest2.us.to/en/developer-documentation

How about we add external links to the Developer Documentation page?

In the future, we would likely add more Specs and Cookbook content. Meanwhile, it seems to me that we could put this blank space into good use and still provide relevant content to the visitor (and not push v0.1 with a mostly empty layout).

Suggestions of links that could be added is welcome.