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ARROW-4216: [Python] Add CUDA API docs

Also reorganize the API docs into several documents, and add/improve docstrings.

To allow building the docs without CUDA enabled, I added some conditional inclusion logic.  When CUDA isn't enabled, the API docs are still generated but the docstrings are empty.  This seems to be the only sane setting that doesn't produce Sphinx errors, one way or the other.

Author: Antoine Pitrou <antoine@python.org>

Closes #3372 from pitrou/ARROW-4216-cuda-py-docs and squashes the following commits:

80600da5 <Antoine Pitrou> ARROW-4216:  Add CUDA API docs