Support formatted RSS content out of the box

[phd20]

I'd love to see content added to the RSS feed. In particular, I'd want to see the custom blogster components supported in that.

Right now, I'm just returning Markdoc.renderers.html(postContent) as content on the RSS feed. It's mostly fine but custom components like CodeBlock and YouTubeEmbed aren't rendered as normal html. It would be nice to have that supported out of the box.