Documentation system for on-line information and printed output

 Texinfo is a documentation system that uses a single source file to
 produce both on-line information and printed output.
 .
 Using Texinfo, you can create a printed document with the normal features
 of a book, including chapters, sections, cross references, and indices.
 From the same Texinfo source file, you can create a menu-driven, on-line
 Info file with nodes, menus, cross references, and indices.