I have a wierd urge to add Gitlab CI to this so that any tagged commit creates a release containing all assets (PDF, ePub, HTML), and a manual job on any commit does the same without a release (stored in Gitlab Generic Package Registry or job artifacts), and the HTML version is built and deployed to the repository's Gitlab Pages on all master pipelines.

I know it's of little use since you usually install all dependencies on a dev machine and simply run fast scripts locally, but Gitlab CI is really a nice tool to use, and you've got some free build minutes you can use on the Saas instance. Then you can make edits on any machine using the Web IDE, and easily share the result hosed on Gitlab pages.