Add scheduler concept guide (#14637)

* Add scheduler concept page

* Rename scheduling overview

* Fix non-ASCII colon symbols

* Reword scheduler concept page

* Move scheduler performance tuning into scheduling

* Signpost from overview to kube-scheduler, etc

* Add whatsnext section to scheduler concept

* Restructure scheduling concept

Now there's a concept page for scheduling, some of the details
in the performance tuning page can have a better home.

* Omit link to (unwritten) scheduling extensions page

* Drop deprecated / superseded filtering rules

content/en/docs/concepts/overview/components.md

@@ -111,5 +111,8 @@ A [Cluster-level logging](/docs/concepts/cluster-administration/logging/) mechan
 saving container logs to a central log store with search/browsing interface.
 
 {{% /capture %}}
-
-
+{{% capture whatsnext %}}
+* Learn about [Nodes](/docs/concepts/architecture/nodes/)
+* Learn about [kube-scheduler](/docs/concepts/scheduling/kube-scheduler/)
+* Read etcd's official [documentation](https://etcd.io/docs/)
+{{% /capture %}}