replace (?) spell checker

We currently use https://pypi.org/project/sphinxcontrib-spelling/

This plugin relies on [pyenchant](https://github.com/rfk/pyenchant), which is no longer being maintained. Pyenchant cannot readily be installed on Windows.

This doesn't create a problem for Mac/Linux or our CircleCI build, but it creates a error when Windows users `pip install`.

Until we solve that problem, we need to conditionally disable spell check from `conf.py` so that Windows users can at least build locally.

Start in conf.py, where the sphinxcontrib-spelling configuration is defined, and review how pyenchant is loaded during a Windows build. Make spell checking conditional so Windows users can build locally while Mac/Linux and CircleCI retain it. Verify the documentation build on Windows and the existing non-Windows build path.

Written by the indexing model from the issue text.