Add support for page break, section break (next page) and section break (continuous page)

[sballesteros]

We will use that in DS3 to disambiguate between front matter and article body in cases where article body doesn't contain headers (if the front matter ends with an abstract with paragraphs and the body has no section we don't know what is what...).

We can probably turn that into a <hr> with a custom class or dedocx-... attribute.