I want to start learning to document my code. What tools do you guys use? I've seen https://www.gitbook.com/ but their server has been on and off all day and I prefer reliable services.

This project is the reason I'm finally learning to document my code: https://gitlab.com/datwood/...

Javadoc or something similar for the language you are using. Documents classes, methods etc. With all their return values, possible thrown exceptions, and parameters.

Your IDE can use it as well to show method signatures while referencing it.