What are some alternatives?
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
MadCap Flare - Documentation for Any Audience, Language or Format
Doxter - Software for creating, organizing and publishing project documentation
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.