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ARROW-4481: [Website] Remove generated specification docs from site after docs migration

We no longer need to render the format docs as part of the Jekyll site, as they were ported to the master Sphinx project. As a result of this, the instructions to build the website were broken

Author: Wes McKinney <wesm+git@apache.org>

Closes #3620 from wesm/ARROW-4481 and squashes the following commits:

013d13bf <Wes McKinney> Remove generated specification docs from site after docs migration