DOC: Enable sphinx.contrib.spelling

http://sphinxcontrib-spelling.readthedocs.io/en/latest/run.html

Saw this in Carol Willing's talk on Sphinx: https://www.youtube.com/watch?v=0ROZRNZkPS8

We'll want to

- add the extension
- build a wordlist of exceptions
- fix the currently failing items
- fail the doc build when there are exceptions