Fix CI to publish into public/${branch_name} for all branches.

Add a command to auto-generate the index when deploying to gitlab pages, with links to every subdirectory (recursion depth 1). The aim is to have one index.html (always up-to-date) for http://olena.pages.lre.epita.fr/pylene that lists the subdirectories where we publish the doc for each tag/branch/release/version/...