Replace jekyll-feed plugin

[bernhold]

The standard jekyll-feed plugin is strongly connected to the blog post concept at the heart of Jekyll and only generates feed items for new posts. There may be a way to configure it to do something with collections as well (see https://github.com/jekyll/jekyll-feed). But it is not clear that there is a way to accommodate the way we do tutorials. We should find/create a tool that will generate feeds for the way we structure things.

[bernhold]

We should be able to adapt the i-p.o RSS feed for what we need here: https://github.com/ideas-productivity/ideas-productivity.github.io/blob/main/feed.xml