Migrate to pydata-sphinx-theme

MetPy has been on pydata-sphinx-theme for some time, and [here is its theme css](https://github.com/Unidata/MetPy/blob/main/docs/_static/theme-unidata.css). Let's follow sometime. With exactly one test install, this theme seems to build fine in our current CI environment, but I haven't explored any farther than that.