Fix RSS feed issues

- [x] Add R category feed, see https://forum.posit.co/t/setting-up-rss-feed-for-a-category-in-quarto-blog/146489/2 - [ ] Fix image problems - compare https://simplepie.org/demo/?feed=https%3A%2F%2Fwww.openstatsware.org%2Fblog.xml - locally this renders fine with Quarto version 1.4.549 - double check how it looks like in the GH action artefacts