Add collectors main readme to learn (#14370)

collectors/README.md

@@ -3,6 +3,9 @@ title: "Collecting metrics"
 custom_edit_url: "https://github.com/netdata/netdata/edit/master/collectors/README.md"
 id: "collectors-ref"
 sidebar_label: "Plugins Reference"
+learn_status: "Published"
+learn_topic_type: "Tasks"
+learn_rel_path: "References/Collectors"
 -->
 
 # Collecting metrics