All of our API docs are maintained by developers who are happiest in git and markdown - so those files live in GitHub, with the added benefit of being available for anyone to contribute to.
Meanwhile, our Ghost(Pro) documentation is maintained by our support team, and integrations & tutorials are maintained by our marketing team - all people who are happiest with the editor and workflows provided by Ghost.
The beauty of JAMstack is just how simple it is to combine multiple sources of data into a single site. Power, flexibility, and most of all choice.
We're not ramming Ghost down anyone's throat. If you don't like it - you really don't have to use it. The decoupling work we are doing intends to make it possible for people to pick and choose bits of Ghost that they do like, and use them however they want.
I understand why Ghost would want to promote using a headless CMS with a database—it's great PR—but I hesitate recommending this as a slam-dunk solution. For me, generally speaking, all the sites I build now are JAMstack (Jekyll, with a custom Rails-based content editor), and having all the website content in a Git repo is a huge part of that workflow and a major selling point.