After searching and trying for a while, I found a perfect alternative to it.
Website from Source Code
DocFX generates API documentation directly from .NET source code. You can use Markdown syntax in the Triple-Slash-Comments in source code. It even allows you to add additional topics to the API documentation using Markdown files. Of course, pure Markdown files are also supported. You can create your static website containing both conceptual topics and API topics within one command.
DocFX can run on Linux, macOS, and Windows. The generated website is pure static, and can be deployed to Github Pages, Azure Websites or any web host with no additional configurations.
DocFX provides a flexible way to customize templates and themes easily.
AND it is developed with .NET core and used by .NET core docs.
I built my documentation from code and comments with DocFx. Then I published the doc as a static website on GitHub pages. It's small, easy to use and integrate but still powerful.
You may interested in other documentation generators recommended by others:
(The introduction is taken from their official websites and may sound like ads. But I'm not promoting them in any way.)
GhostDoc is a Visual Studio extension that automatically generates XML documentation comments for methods and properties based on their type, parameters, name, and other contextual information.
VSdocman is an ultimate, fast and easy to use .NET documenting tool for every Visual Studio C# and VB developer.
HelpStudio is the fastest, easiest way to create and publish procedures, documentation and help systems and integrate them with your desktop, web or mobile applications or components.
Doc-O-Matic is a software source code documentation and Help authoring tool. Doc-O-Matic takes source code and comments, adds conceptual topics and external input and produces documentation from it.
Monodoc is a set of libraries and applications for viewing and editing Mono class library documentation.
Generate API documentation in multiple formats from XML comments and .NET libraries, fast and easy. View documentation live. Free and open source.