Plain markdown files is definitely the way to go. It's easy to create a basic table of contents and arrange them in a tree-like manner. You can view them render

Plain markdown files is definitely the way to go. It's easy to create a basic table of contents and arrange them in a tree-like manner. You can view them rendered in a GitHub project. If you want search and some other features, consider the mdbook project used by Rust. It's not specific to Rust and it's pretty slick. I have used docusaurus, vuepress, mkdocs, antora, and asciidoctor for documentation. I like mdbook the best.