Publish all non dont_publish.* branches in branches/ subdirectory

This MR makes the site be generated for every branch, the name of which does not begin by dont_publish, and the generated site can be browsed in the subdirectory branches/$branch_name.

For instance, for this MR: https://tmartine.gitlabpages.inria.fr/doc/branches/publish_branches/

It is worth noticing that page generation can be enabled for forks (in the example above, it is on my personal fork): the runner sed-paris-ubuntu-18-04-inria-ci is shared, it suffices to enable it in CI/CD settings.