Add a link to the online doc in the readme of the doc

doc/README.md

+If you only want to get the documentation, note that a pre-built
+version for the latest release is available
+[online](http://skmad-suite.pages.lis-lab.fr/madarrays/).
+
 Sphinx is used to generate the API and reference documentation for all
 packages from the [`skmad-suite`](https://gitlab.lis-lab.fr/skmad-suite/).