What do you use to write documentation? LaTeX? Word? Markdown?

Latex.

Almost all of the source code for Axiom (1.2 million lines of code) lives inside latex documents. They are literate programs. I can explain the most complex ideas along with the code that implements them. And I can illustrate the ideas using images, diagrams, and figures.

Org-mode in Emacs. Because it enables to do literate programming.

In this video [0] by Howard Abrahams you can see how to wave the instructions and the execution of them with Emacs.

Another advantage is that you can mix your diagrams (in plantuml or ASCII art) with your code, in the same org-mode file.

Gitub displays .org files the same way it does Markdown.

[0] https://youtu.be/dljNabciEGg

I find myself reaching for LaTeX for complex documents that I also want to look great. Curious to hear what others use.

Markdown and LaTeX.