Proposal: Improve navigation with new theme

[mradamcox]

describe the issue

This ticket is a proposal to switch themes from the default RTD theme that we've been using to something new with better sidebar navigation and a better capacity for displaying large amounts of content for different audiences, like we do. For example, it would be nice to have more general headers in that menu with expandable content below them. This looks like a good example:

https://pydata-sphinx-theme.readthedocs.io/en/latest/demo/index.html

Also, it may be nice to "freshen" up the documentation with a new theme.

To try this out, a new branch should be made that uses the theme, and that branch should be published to Read The Docs on a hidden build.

which release does this issue concern?

6 + (no need to backport a new theme to older releases.

[dwuthrich]

@mradamcox I like the idea of improving the documentation theme to improve navigation and presentation. Your example looks like a significant improvement over our current UI