Draft: Add information and helper scripts for creating releases

The information and scripts come from

https://gitlab.dune-project.org/infrastructure/releases.git

which I suggest to abandon. The dune-website sources need to be touched by release managers anyway, and having all relevant information there is more convenient than having it scattered across various tiny repositories.

Also, even now there is considerable overlap between the scripts in dune-website/bin and the releases repo.

Marked as 'draft' between the text needs a bit of cleanup.