Add documentation about CPU arch compatibility (#4302)

docs/arch.md

+## Architecture compatibility
+
+The following table shows the impact of the CPU architecture on compatible features:
+- amd64: Cluster using only x86/amd64 CPUs
+- arm64: Cluster using only arm64 CPUs
+- amd64 + arm64: Cluster with a mix of x86/amd64 and arm64 CPUs
+
+| kube_network_plugin | amd64 | arm64 | amd64 + arm64 |
+| ------------------- | ----- | ----- | ------------- |
+| Calico              | Y     | Y     | Y             |
+| Weave               | Y     | Y     | Y             |
+| Flannel             | Y     | N     | N             |
+| Canal               | Y     | N     | N             |
+| Cilium              | Y     | N     | N             |
+| Contib              | Y     | N     | N             |
+| kube-router         | Y     | N     | N             |