add an RSS feed of available dataset versions, linked from dataset.html

We should complement the dataset tables at https://docs.softwareheritage.org/devel/swh-dataset/graph/dataset.html with an RSS feed, containing all available versions, of both complete and teaser datasets. It can be generated from the same information we have in the tables on that page (e.g., we can just use the dataset timestamp as event timestamp). This will allow people to stay up-to-date with dataset releases, and possibly even automate downstream data processing flows on top of them.