As I read through these comments (and discover some interesting tools!) I want to give a word of caution for anyone using this thread to make a tooling choice. The "best" documentation system for you is heavily dependent on your product [1], your contributors [2], and your ability to maintain/customize the underlying tooling. Just because you're seeing Tool X 10 times in this comment doesn't necessarily mean it's best for you!

[1] E.g. documenting an API is very different from documenting a GUI product

[2] E.g. on https://web.dev we had a large pool of authors. They were all fairly technical but even so we often ran into questions about build errors, etc.