add link to RSS feed

By making sure we emit all alternate output-formats as re-links, we make
sure that RSS-plugins etc will offer the user to subscribe to the feed.

Without this, the RSS feed is quite hidden.
2 jobs for master in 17 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #2997078
pages

00:00:17

 
  Deploy
passed pages:deploy

00:00:00