Do not recompile all branches all the time, use browsable artifacts

Suggested by @khuynh.

This merge request stops recompiling all the branches but uses browsable artifacts. For instance, the result of this PR can be seen here: https://tmartine.gitlabpages.inria.fr/-/doc/-/jobs/2367465/artifacts/private/index.html

More generally, to see the result of a commit, go to the log page of the job in the commit's associated pipeline (for instance, here: https://gitlab.inria.fr/tmartine/doc/-/jobs/2367465): when the job is completed, in the section "Job artifacts" at the right hand side, there is a link "Browse". Then, go to the folder private/ and choose index.html.

This merge request also updates ananke theme and fix the deprecated usage of .URL to be compilable with the last version of hugo.