http://devhub.wmflabs.org is a prototype of the "Data and developer hub", a portal and set of pages whose goal is to encourage third-party developers to use Wikimedia data and APIs. Check it out. Your feedback is welcome, you can discuss in this mail thread, or file Phabricator tickets in the project dev.wikimedia.org.
Around the 2014 Zurich Hackathon, Moiz Syed, Juliusz Gonera, and others discussed creating "a thing" to expose our APIs and data to developers. When S Page moved to WMF tech writer, he wrote some articles for this and with Quim Gil developed a landing page from the wireframes.
The prototype is using the Blueprint skin and running on a labs instance, but the articles are all regular wiki pages on mediawiki.org.
Thanks to everyone who participated in the gestation of this idea.
S Page and Quim Gil
Q: How can I get it to feature my API?
A: Create a task in dev.wikimedia.org and documentation with "Article" in the title. You can write an article yourself, see dev.wikimedia.org/Contributing.
Q: Yet another site? Arghh!
A: Agreed, T101441 "Goal: Integrate new Developer hub with mediawiki.org". It's a separate site for now in order to present a different appearance.
Q: But why a different appearance? Why a separate skin?
Our competition for developer mindshare is sites like https://developers.google.com/. We believe looking like a 2000s wiki page is a deterrent to using APIs and data. We hope that many third-party developers join our communities and eventually contribute to MediaWiki, but "How to contribute to MediaWiki"  is not the focus, providing free open knowledge is.
Q: Why the Blueprint skin?
A: Prateek Saxena, Andrew Garrett, and May Galloway developed it for the OOUI Living Style Guide  and it has some nice features: a fixed header, and a sidebar that gets out of the way and combines page navigation and the TOC of the current page.
Q: So why not use the Blueprint skin on mediawiki.org?
A: Agreed, T93613 "Deploy Blueprint on mediawiki.org as optional and experimental skin" is a blocker for T101441. We appreciate help with it and its blockers.
Q: I hate the appearance.
A: That's not a question :) Then forget the prototype exists and view the same content at https://www.mediawiki.org/wiki/API:Data_and_developer_hub
Q: Is this "dev.wikimedia.org"?
A: Sort of. http://dev.wikimedia.org will be the well-known shortcut to the "hub" page. And dev.wikimedia.org is the project name for this "Data and developer hub".
Q: I thought dev.wikimedia.org was going to integrate generated documentation/replace doc.wikimedia.org/enumerate all Wikimedia software projects/cure cancer, what happened?
A: One step at a time. For now, its goal is, to repeat, "to encourage third-party developers to use Wikimedia data and APIs".
Q: Why are the pages in the API: namespace?
A: That's temporary, they will probably end up in a dev: namespace on mediawiki.org that uses the Blueprint skin by default (T369).
Q: Where are the talk pages?
A: It's a bug that the sidebar doesn't have a "Discussion" link. The talk pages on the prototype all redirect to the talk pages on mediawiki.org, and Flow is enabled on them.