While having HTML package manuals is a good way for users to easily access documentation on demand, GitHub's wikis feature might provide a more centralised method for users to not even have to leave the repository to learn about the package.
Currently, Wikis seems to support locally editing markdown files to be published to the repository, and can be cloned. Of course, this may lose out of some of the HTML features that make GAP documentation unique, so may be suited for a selected subset of AutoDoc comments (e.g. development/usage guides).