Page subtitle for root level 1 export should honor assembly-navtitle

Originally, the assembly-navtitle override was implemented for pages included into an assembly (essentially chapters and sections). See #109 (closed). And it even influences the generated filename if defined on a page referenced by a top-level nav entry. However, it does not change the doctitle of that document. That requirement was missed.

Expand the assembly-navtitle feature so it influences the doctitle of the assembly taken from the top-level nav entry when root level is 1.