You can checkout Ghost Theme Marketplace here:
https://royce.justgoodthemes.com/ - could be an option (side menu), but is more blog style rather than documentation, but with some dynamic routing you could probably make it work.
https://editorial.ghost.io/ - could also be an option
https://paperleaf.nvslbs.com/ - also a possible option
Or you can search on themeforest.net for a Ghost theme that suits your needs.
I don’t think there’s a theme ready made for doc sites purposes except docs.ghost.org theme, but that’s a bit more complicated and works on gatsbyjs as a frontend.
But you can always ask the theme developers on this forum to give you a hand on creating such a theme.