Make mTLS tutorial consistent

Added the 'Prerequisite' section to the tutorial to make it consistent with the others.

docs/quick-tutorials/k8s-mtls.mdx

@@ -15,7 +15,21 @@ In this tutorial, we will:
 - Deploy client and server pods communicating over HTTP with mTLS.
 - See that mTLS credentials were autogenerated.
 
-We'll start by installing Otterize. You can do so just using Otterize OSS, without it to Otterize Cloud, and use SPIRE for certificates. Or you can do so with Otterize OSS connected to Otterize Cloud, which adds web visualization as well as the *option* of using Cloud-managed credentials. (You can also connect to Cloud but still use SPIRE; refer to the [cryptographic credentials documentation](/security#cryptographic-credentials).)
+## Prerequisites
+
+<details>
+<summary>Prepare a Kubernetes cluster</summary>
+
+Before you start, you'll need a Kubernetes cluster. Having a cluster with a [CNI](https://kubernetes.io/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/) that supports [NetworkPolicies](https://kubernetes.io/docs/concepts/services-networking/network-policies/) isn't required for this tutorial, but is recommended so that your cluster works with other tutorials.
+
+{@include: ../_common/cluster-setup.md}
+</details>
+
+You can now install Otterize in your cluster (if it's not already installed), and optionally connect to Otterize Cloud. Connecting to Cloud lets you:
+1. See what's happening visually in your browser, through the "access graph";
+2. Avoid using SPIRE (which can be installed with Otterize) for issuing certificates, as Otterize Cloud provides a certificate service.
+
+So either forego browser visualization and:
 
 <details>
 <summary>Install Otterize in your cluster, <b>without</b> Otterize Cloud</summary>