Remove bulma subheading styling

[jarrodmillman]

See the subheading at the top of https://scientific-python.org/summits/developer/2023/ in dark mode.

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[stefanv]

I think it would be good to keep some styling for the subtitle, since PDST doesn't have subtitles. The following in styles.css should suffice:

.subtitle {
  font-style: italic;
  margin-top: -0.5rem;
}