[ENH]: hi-res plot directive... #25497

Problem

Our plot directive makes 100-dpi figures for the webpage. These look pretty fuzzy on hiDPI screens, and we should do what we did for sphinx gallery and allow hi-res figures..

Proposed solution

Not quite sure how to fix this. We currently make a .. figure:: from the plot directive, which is exactly the same as a .. image:: except it allows a caption. Internally, we use the caption functionality exactly once. If we could drop the caption we could just use .. sg-image:: from sphinx gallery, which allows srcset multiple resolution images.

Just increasing the dpi doesn't really work because it makes the images twice as big, but still low resolution, unless we have specified the :width: manually.