I find that the best way to build docs is to use some sort of a static html generator and host the html yourself, which is not at all hard to do. There are free and open source themes available to host your own docs in various site generators.

My personal favorite is hugo. Previously I have built a documentation for a large project [0] using hugo and it gave us 100% control over how we design or host it. Outsourcing a doc to a SaaS seems like something I would never do if I cared about the product/software.

[0] - https://docs.dgraph.io