arc42 [1], rendered into whatever format you prefer for reading.
In our case we write asciidoc (sometimes markdown) and render it to HTML for each of the releases, so that a version of the documentation is delivered with the release.
The authors of arc42 also encourage users to set documentation under version control to ensure one can keep the project and its documentation in sync [2].
[1] https://arc42.org/overview [2] https://faq.arc42.org/questions/G-1/