API Sites: Ability to publish pre-release docs

[scottgonzalez]

Right now we don't publish API docs until there's a stable release. We should be able to publish docs during the pre-release phase and still have the unversioned URL show the stable docs.

[fnagel]

Symfony does a nice job on making there doc versions available (note the edit button and the version select) : http://symfony.com/doc/current/cookbook/index.html