docs: link out to a blog post about version skew

docs/howto/upgrade-cluster/k8s-version-skew.md

@@ -5,7 +5,7 @@ When we upgrade our Kubernetes clusters, we upgrade the k8s control plane (k8s
 api-server etc.), the cloud providers managed workloads (`calico-node` etc.),
 and the k8s node's software (`kubelet` etc.). Since these upgrades aren't done
 at the exact same time, there are known constraints on how the various versions
-can _skew_ in relation to each other.
+can [_skew_] in relation to each other.
 
 When we upgrade, we are practically constrained by [Kubernetes' version skew
 policy] in the following ways:
@@ -18,4 +18,5 @@ policy] in the following ways:
 2. Nodes' k8s version must not be newer than the control plane, and be at most
    three minor versions older (`kubelet` requirement).
 
+[_skew_]: https://www.industrialempathy.com/posts/version-skew/
 [Kubernetes' version skew policy]: https://kubernetes.io/releases/version-skew-policy/#supported-version-skew