Add concept page about Controllers (#15733)

* Add controller concept

* Fix controller glossary entries

* Link from Understanding Kubernetes Objects to Controllers

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

@@ -115,6 +115,7 @@ saving container logs to a central log store with search/browsing interface.
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 * Learn about [Nodes](/docs/concepts/architecture/nodes/)
+* Learn about [Controllers](/docs/concepts/architecture/controller/)
 * Learn about [kube-scheduler](/docs/concepts/scheduling/kube-scheduler/)
 * Read etcd's official [documentation](https://etcd.io/docs/)
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content/en/docs/concepts/overview/working-with-objects/kubernetes-objects.md

@@ -73,6 +73,7 @@ and the `spec` format for a `Deployment` can be found
 
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 * Learn about the most important basic Kubernetes objects, such as [Pod](/docs/concepts/workloads/pods/pod-overview/).
+* Learn about [controllers](/docs/concepts/architecture/controller/) in Kubernetes
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