How to update Sphinx options during the build

from blog Chris Holdgraf's blog, | ↗ original
As part of [the `pydata-sphinx-theme`](https://github.com/pydata/pydata-sphinx-theme/pull/1075) we have a few settings that auto-enable extensions and configure them on behalf of the user. It has always been mysterious to me how to do this properly **during the Sphinx build**. It's easy to configure things with `conf.py` ahead of time, but what...