Ask HN: What do you use to manage your product's documentation?

Are there any good CMS's for documentation? How do you keep track of slightly different versions of a text that are served in different places? (eg. I might want to tailor the wording or add a few paragraphs in the Setup Guide to be a bit less terse but still use the same code snippets.) Or just what do you use to manage your product's documentation? (I'm not interested in "projects" that might just have a file somewhere on Github but products, with sites dedicated to selling them, with setup guides, api references, tutorials, etc.)