Gitbook is a modern documentation platform where teams can document everything from products to internal knowledge-bases and APIs. As said, your documentation is really key for yourself, your users and everyone in your ecosystem. And you should document everything and make it available easily, that's why I took the time to describe this tool, originally an Open Source software but currently the team is focused on making it available through a platform in SaaS mode.
Previously, I was using the gitbook cli but it is now deprecated so you must see the next section on using the gitbook platform. In case you would like to use the cli, please follow the Setup and Installation of GitBook
As said previously, gitbook focus is now on the platform itself, so go to the sign-up page. In term of pricing, you can select the Personal option: A place to share your personal projects and notes which include 10 spaces and a custom domain.
If you're using Github, I will recommend then connecting your space with your repo, so follow the steps describe in GitBook.com / GitHub integration.
The benefits of using the cli is that you can also export your documentation in eBooks and PDFs and also this is available as HTML pages. If you run gitbook build command, you will see that all MD files have been converted in HTML format under the _book directory.