My recommendations are:
- MkDocs: Good default choice, reasonably flexible.
- Jekyll: For people who want a little more flexibility—things like landing pages, blogs, etc.
- Antora: For people who want the best docs, and are willing to put in the most effort. It will manage, for you, the process of generating documentation sites that collect documentation from multiple projects and possibly multiple versions of each project. Asciidoc is full of features you’ll find useful.
Hugo looks like it has a lot of flexibility like Jekyll, but it seems to take more effort to get everything working the way you want, and it also seems like there’s just too much variation in how the different themes work. To be honest, I never really managed to make anything with it—I found out that the theme I was using didn’t have some features I wanted, so I switched to a different theme, but it’s not easy to switch themes. I was too frustrated and gave up.
It looks like MdBook is reasonably active, so I’m sure it will catch up.