DOC: Render field list headings in the same style as rubrics

[seisman]

Render the headings of field lists (e.g., "Parameters", "Returns") in the same style as rubrics (e.g., "Examples", "Notes"), instead of in a grey box.

Implemented by Claude Opus 5.5, reviewed by @seisman.

Preview:

• https://pygmt-dev--4941.org.readthedocs.build/en/4941/api/generated/pygmt.binstats.html#pygmt.binstats
• https://pygmt-dev--4941.org.readthedocs.build/en/4941/api/generated/pygmt.Figure.basemap.html#pygmt.Figure.basemap

[seisman]

With changes in this PR, now "Parameters"/"Examples" all have the same stype, as shown below:

However, it seems that both sphinx_rtd_theme and sphinx_pydata_theme use different styles for "Parameters" and "Examples", e.g., https://pandas.pydata.org/docs/reference/api/pandas.api.typing.Resampler.apply.html.

Reading https://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html#confval-napoleon_custom_sections, it seems it's intended that "Parameters"/"Returns" have different styles, compared to other section headings.

One thing that I like sphinx_pydata_theme is that, it shows the parameter name and description on separate lines, e.g.,

which is more readable than the sphinx_rtd_theme's behavior, e.g.,