What document writing tool/s would you recommend for a beginner SaaS dev

I am about to finish my first bigger project (ever). I was wandering what tool is considered the standard in writing technical documentation? I use word, because it is mainstream easy and I have anyway a license for the whole office 365 package. But somehow I feel I am doing this wrong. A friend of mine (and a long time dev mentor) suggested that Sphinx is a good alternative. I have nothing against markup and my main training was in Pythong anyway. But still... something is missing. :)