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Review of DocFX powered by the Slant community.
DocFX can generate documentation from source code (.NET, RESTful API, JavaScript, Java, MarkDown etc...), it seems to be the successor of SandCastle.
See article on Medium Create Api Documentation with DocFx using Visual Studio and CLI.
SpecsUpdate
PlatformsWindows, Linux, Mac
Output FormatHTML, PDF
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Pros
Pro .NET Core
Developed in .NET Core.
Pro Free and open source
Cons
Con No CHM support
Does not seem to support .chm help file format, only HTML and PDF.