h2/second-level sections are overlined to separate them from one
another. However if the document has no introductory paragraph (text
between the h1/document title and the first subsection) the h1 has an
underline and the section's overline, which isn't great.
Fix CSS so a 2nd-level section which immediately follows an h1 does not
get the overline.
After discussion with fp this probably isn't the direction (ish), but
want to checkpoint this in order to work on other stuff earlier in the
document, but not commit a single gigantic blob of stuff.
