Use latest pydata-sphinx-theme

[travishathaway]

What is the idea?

The primary dependency of this project, pydata-sphinx-theme, has been making considerable improvements, and it would be worth it to bump the version we depend on to the latest version of that project.

What should happen?

We've made a couple changes that differ from the main project. The trickiest of which is the fact that we maintain our on layout.html. As we move to update to the latest version, we need to make sure that everything works. It might even be better to begin by copying over the latest version of this file directly from that repository or see whether we can avoid have to maintain our own version altogether.

Additional Context

This was done in order to have a sidebar menu that always appears on the website, especially the home page. If we can find another way to do that that does not involve maintaining our own version of layout.html, we should do it.