All documents are in the components of the corresponding repositories in the catalog doc/in markdown format. For example, you can find "framework" node_modules/aurelia-framework/doc/articledocuments in the section and binding documents in node_modules/aurelia-binding/doc/article.
As far as I know, the code for the hub itself is not included in github, but you can use raw markup files that also appear in the hub. In fact, you can even find them so that from time to time there is more information about the hub when the hub is not yet updated with the latest changes.