Add "Documentation" entry in the menu

As suggested on the mailing list. This documentation is probably part of the most important content served on bitcoin.org so adding it to the menu seemed like a logical step.

![capture du 2014-05-29 14 19 16](https://cloud.githubusercontent.com/assets/3578089/3122725/93ac7a22-e76a-11e3-932f-6119b9caaceb.png)

Consistently with other English-only pages, devel-docs is only displayed in the English menu (maintaining translations for devel-guide and devel-ref seems unfeasible right now).