Change docs theme to Pydata Sphinx theme

The PyData Sphinx Theme has received a lot of updates recently and it's looking awesome, with lots of momentum. Let's use it!

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

Start by locating the repository's documentation configuration and current Sphinx theme, then read the linked PyData Sphinx Theme documentation for setup requirements. Done means the project documentation builds successfully and uses the PyData theme consistently; the issue does not name specific files or tests.

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