Add RSS feed of new tutorials

Some devs may not want to hand over their email addresses, so we could offer an RSS/Atom feed instead.

There is a [Jekyll plugin](https://github.com/jekyll/jekyll-feed/blob/master/lib/jekyll-feed/feed.xml) that offers an Atom feed. It would need configuring properly; when I tried it no articles were included. I think this may because it iterates over `posts`?

Start by inspecting the Jekyll configuration and tutorial content collections, then compare them with the jekyll-feed plugin's feed.xml implementation. Run a site build and check why no articles are included. Done means the generated RSS or Atom feed contains the new tutorials and is configured for the site's content.

Written by the indexing model from the issue text.