Link KDD upgrade instructions in the upgrade from MKE-3 section

[ry4nz]

We have instructions to allow customers to upgrade from etcd Calico backend to KDD in MKE-3, let's link those here so it's easier for customer to navigate.

[KoryKessel-Mirantis]

@ry4nz, please clarify the ask here. What I get is that we have content that instructs customers on how to upgrade from etcd Calico backend to KDD at https://mirantis.github.io/mke-docs/docs/migrate-from-mke-3/. What I do not have here is where we want to drop the link?

[moshiurH]

Do we mean to say include links in the MKE 3 docs on how to switch from etcd to KDD calico backend? Do we want to add the MKE 3 to 4 migration guide in MKE 3 docs?