Add common layout elements for RSS

[savq]

I've noticed that many Julia blogs enable RSS, but don't actually make it discoverable because their heads don't include metadata indicating the site has a feed. Something like

<link rel="alternate" type="application/rss+xml" title="Feed" href="https://example.com/atom.xml">

Adding this metadata to the templates would help authors be more aware of this issue, and it would help readers subscribe more easily to the feeds. If I don't see the feed icon in my browser, I'll usually assume the site doesn't have one.