Adds news post for RSD launch

This is the news post for the RSD launch that is scheduled for today.

@jandt-desy @konrad @frust45 It would be great to hear you opinion about the post.

I could not figure out how to use breakpoints so that float-right is only for larger screens, e.g. > md.

Review link

_data/navigation.yml

@@ -80,6 +80,8 @@ header:
     entries:
       - title: "News"
         url: "news/"
+      - title: "Newsletters"
+        url: "newsletter/"
       - title: "Upcoming events"
         url: "https://events.hifis.net/category/14/"
         external: True