Add RSS link to displayed content of announcements blog page

[fredcy]

I'm a new NixOS user and I would like to follow NixOS announcements via RSS.

To find the RSS feed for https://nixos.org/blog/announcements I had to read the page source (and ask on matrix), which is awkward.

I suggest adding explicit mention of the RSS feed to the content of that blog page. I expect to find such a thing by spotting the RSS feed icon, or by searching for "feed" in the content.