Split up doc Rakefile to minimize dependencies

[elsbrock]

Please consider splitting up the Rakefile for doc generation, because many of the dependencies are not needed for the generation of the manpage. Specifically, the following libraries are unused for the task manpage:

• bonsai
• liquid
• maruku

On Debian, bonsai is not available as a package, and as long as it's not required for the manpage generation, I'd prefer not to be required to package it.

[nicowilliams]

@else I'm not really conversant with the doc building side of jq. Unless @stedolan wants to do this or you submit a PR, I don't think it's likely to happen soon. But I agree that doc building has to be easier.