Try alternative Sphinx themes for the PyGMT documentation?

The PyGMT documentation has used the [Sphinx RTD theme](https://github.com/readthedocs/sphinx_rtd_theme) since its first commit. It has worked well, and its relatively infrequent releases (xref: https://github.com/readthedocs/sphinx_rtd_theme/tags) can be viewed as a good thing (stable behavior and little maintenance burden) or a bad thing (missing documentation features).

It may be worth experimenting with alternative themes from the [Sphinx Themes gallery](https://sphinx-themes.org/). One popular option is the [PyData Sphinx Theme](https://pydata-sphinx-theme.readthedocs.io/), which is used by [NumPy](https://numpy.org/doc/stable/), [pandas](https://pandas.pydata.org/docs/), [xarray](https://docs.xarray.dev/en/stable/), and other projects in the PyData ecosystem. It offers a more flexible layout, light/dark mode, and configurable navigation.

I'm not proposing that we replace the current theme. Instead, this issue invites anyone interested to try an alternative in a small proof-of-concept PR and share a documentation preview or screenshots. We can then discuss whether a theme change would be worthwhile.

Start by locating the current Sphinx RTD theme configuration in the documentation setup and compare it with a candidate theme from the Sphinx Themes gallery, especially PyData Sphinx Theme. A proof-of-concept is done when a PR provides a documentation preview or screenshots and enough comparison for the project to discuss whether changing themes is worthwhile.

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