Wiki for your SaaS?

Have a look at notaku, documentation based on notion, currently lifetime deal on pitchground.

How about something like GitBook?

Would use Notion because it's simple, WYSIWYG and simple to publish as public pages for users.

i write everything in one page in org mode in emacs and than generate single page html.
my project is small enough that it works here is the documentation output https://newbeelearn.github.io/sserver/ . You can look at index and css in same github repository as well.

I don't currently have any public documentation for our current project, but personally knowing Vue, I've had good success with VuePress in the past.

I think it probably depends on your tech stack though, and whether or not you're looking to include the documentation within your main site vs. having something standalone, as that may guide you down a specific route.