API Sites: Provide offline access

[jzaefferer]

We currently provide api.xml, but that's not intended for users to read. Since we're dropping docs in jQuery UI's download builder, we want to provide an alternative on the API sites for offline reading, for all sites, not just jQuery UI.

We should look at third-party sites that work offline. http://devdocs.io/ is pretty nice, but currently requires cloning the repository to use it offline. Though the about page says "An offline version that requires no setup is planned for the future.", may be we can work with them to speed that up?

Dash is OSX-only, but provides jQuery UI docs, among others, and works fine offline: http://kapeli.com/dash