When I started an open source project, I spent quite a bit of time researching various documentation options. Initially I went with AsciiDoc, but later switched to MkDocs with the Material theme. I really like MkDocs. It is easy to set up (everyone has python installed), has a lot of features and configuration options. The Material theme looks awesome out of the box. I didn't have to mess with CSS. It's definitely worth a try as it is very quick to set up.