Dr.Explain - Dr.Explain software is a help file authoring tool.
Greenboard - Generate beautiful static api documentation
MadCap Flare - Documentation for Any Audience, Language or Format
You Need A Wiki - Create a wiki with Google Docs
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Docusaurus - Easy to maintain open source documentation websites