We're looking to adopt the Pydata sphinx theme (https://pydata-sphinx-theme.readthedocs.io/en/latest/) as the new theme for Rubin's Sphinx documentation projects (note that technotes are a separate matter, requiring a specialized theme). The PyData Sphinx theme has a nice responsive design, light/dark theme, uses CSS variables for colours. Most distinctively, it has a concept of "parts" links from the theme's top navigation bar. This helps us support very large documentation sets.

In this ticket we'll use Documenteer's own documentation as a test bed for integrating and customizing the PyData sphinx theme. We'll create a reusable Sphinx configuration for Rubin documentation sites to share this theme configuration. As the issue progresses, we'll create additional tickets for significant new areas of work that are uncovered. In particular, updating the "stack" documentation (i.e. pipelines.lsst.io) will be a separate issue.