ReadTheDocs is a popular documentation renderer direct from sourcecode. Github uses it a lot and many open source projects rely on it. A brief intro, everything is kept with the repository and whenever there is a push/tag readthedocs server start a build of your documentation using Sphinx. This means that your documentation is 100% up-tp-date with the code repository.
I have recently created a ticket on ReadTheDocs github repository for Gitlab integration. It looks like they have done and merged the hook into their repository. I would be great to have ReadTheDocs as a service in the service page.