For paid options, process street is undoubtedly the best. For libre it depends on if you need PDF export and whatnot. If that is needed then Sphinx has all the bells and whistles and can be used with readthedocs if you want it hosted for free (publicly visible). The other options have to be hosted in some way on a VPS etc. For something light that doesn't need PDF export then docsify or mkdocs is probably the best. For something that outsiders can edit and that does pdf/epub export than gitbook is probably the best. I think ultimately gitbook is the best middle of the road for libre but you need a vps.
Process street https://www.process.st/ - Paid but very simple and has features none of the other ones have, like the ability to create a new instance of a process workflow and go through it (onboard a new student etc)
Slate https://github.com/lord/slate - more focused on coding development
Docusaurus https://docusaurus.io/ - pretty good. hooks in with algolia search which is very good for searching.
Booksta