Allow disabling of `api_reference`

[skwerlman]

I'm generating different subsets of my docs depending on the Mix.env; in dev i'm generating docs for all modules, while in prod i only want to generate docs for a set of guides i've written.

This means that all applications are ignored, so I wind up with an empty api-reference.html linked from all my pages, and there seems to be no way to remove it.

Ideally, I'd like a config option like api_ref: false or similar that i could specify in mix.exs to prevent that page from being generated. Alternatively, if the page were not generated if it would be empty, that would also solve my issue.

[josevalim]

Yes, please do send a PR.

[skwerlman]

How do we want to handle cases like not_found_template.eex when api-reference is disabled? just remove the text referencing it? or do we want to link somewhere else?

[josevalim]

Closing in favor of #899.