[max-models]

The "Web figures" guide now shows embedded web figures, not only describes them.

• The guide becomes MDX, with an Examples section of three figures embedded with PlotlyFigure, each with its code:
  • lines in the site's palette with a black reference line, from a Canvas;
  • a Plotly heatmap with a contour in Matplotlib's C1: the colormap stays and the contour takes the palette;
  • an animation with a slider and Play button.
• scripts/web_figure_examples.py writes them to docs/public/figures/. The docs workflow runs it as pre-build, and the smoke paths check the page and the three files. Run it by hand for astro dev.

Two fixes to the Plotly output of a canvas, found on the way:

• An untitled subplot no longer gets the placeholder title "(0, 0)".
• The margins were 10 px on every side, which cut off the axis titles. A web figure's automargin makes room for tick labels, but Plotly then pins a title without a standoff to the edge of the figure, half outside. The left and bottom margins now leave room when there is a y or x label (70 and 60 px), and the axis titles get a standoff of 8 px.

Checks:

• I looked at the page in the dev server in light and dark mode. The Play button and slider need PlotlyFigure: buttons and sliders in the site's colours starlight-theme#57; without it they stay white in dark mode.
• 341 tests pass, including a new one for the margins and the title.
• The pre-commit hooks pass, and starlight-check-mdx compiles all 22 MDX files.