I used doxygen and I've seen various others that I can't all remember. I just saw this, which seems tailored to .NET (and thus should work well with Unity code).
https://dotnet.github.io/docfx/
But there's always room for a new tool, especially if it's tailored to a specific language or framework. A long time ago I wrote an HTML doc exporter for Common Lisp code because I wanted to generate information from runtime (function doc strings and class hierarchies, for example). If you package yours up to conveniently install from the Unity Asset Store and run from the Unity Editor, I think there'll be interest in it.
For a deep historical dive, PaulsGames mentioned LaTeX, which is descended from TeX, which was part of Donald Knuth's push for "literate programming"