`module-heading` missing styling

[alexlafroscia]

The class is assigned, but there's no CSS for it. This ends up leaving the result looking pretty awkward.

[alexlafroscia]

I tried replacing it with a {{docs-viewer/x-section} but the vertical spacing is really weird.

[alexlafroscia]

I think this might be a good use-case for indenting subnav as suggested in #126; each module's stuff could be indented (or rendered in whatever way a subnav is visually set apart from whatever else is in the list).

[pzuraq]

Agree that this should overlap with the work from #126, though I think the styling should be even slightly different for these items, to distinguish them from class names/functions