Skip over intro paragraphs in the SxS

The parser assumes the first thing it's reading is a header. Not all SxS sections start this way, though, which proved to be a particular problem when those intro paragraphs included citations. The parser treated the paragraphs as a header, found their labels, and duplicated their children (once per citation.)

Since intro paragraphs are at the begininning, this causes the SxS structure to be gigantic.