Add links out to different metrics APIs (#14865)

There's been a number of questions around the difference between the external.metrics.k8s.io and custom.metrics.k8s.io in #sig-autoscaling referring back to the HPA docs recently. Added links out to the design proposals for each and the relevant sections of the existing walkthrough docs.

content/en/docs/tasks/run-application/horizontal-pod-autoscale.md

@@ -271,6 +271,14 @@ APIs, cluster administrators must ensure that:
 
 * The `--horizontal-pod-autoscaler-use-rest-clients` is `true` or unset.  Setting this to false switches to Heapster-based autoscaling, which is deprecated.
 
+For more information on these different metrics paths and how they differ please see the relevant design proposals for
+[the HPA V2](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/autoscaling/hpa-v2.md),
+[custom.metrics.k8s.io](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/instrumentation/custom-metrics-api.md)
+and [external.metrics.k8s.io](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/instrumentation/external-metrics-api.md).
+
+For examples of how to use them see [the walkthrough for using custom metrics](/docs/tasks/run-application/horizontal-pod-autoscale-walkthrough/#autoscaling-on-multiple-metrics-and-custom-metrics)
+and [the walkthrough for using external metrics](/docs/tasks/run-application/horizontal-pod-autoscale-walkthrough/#autoscaling-on-metrics-not-related-to-kubernetes-objects).
+
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