Document Eventing istio integration (#5879)

Signed-off-by: Pierangelo Di Pilato <pierdipi@redhat.com>

Author: pierDipi

config/nav.yml

@@ -288,6 +288,7 @@ nav:
         - EventType auto creation: eventing/features/eventtype-auto-creation.md
         - Transport Encryption: eventing/features/transport-encryption.md
         - Sender Identity: eventing/features/sender-identity.md
+        - Eventing with Istio: eventing/features/istio-integration.md
       - FAQ: eventing/faq/README.md
     # Eventing reference docs
       - Reference:

docs/eventing/features/README.md

@@ -75,3 +75,4 @@ Knative Eventing:
 | [New trigger filters](new-trigger-filters.md)              | `new-trigger-filters`  | Enables a new Trigger `filters` field that supports a set of powerful filter expressions.                                                                                                                          | Beta, enabled by default   |
 | [Transport encryption](transport-encryption.md)            | `transport-encryption` | Enables components to encrypt traffic using TLS by exposing HTTPS URL.                                                                                                                                             | Beta, disabled by default  |
 | [Sender Identity](sender-identity.md)                      | `authentication-oidc`  | Enables Eventing sources to send authenticated requests and addressables to require authenticated requests.                                                                                                        | Alpha, disabled by default |
+| [Eventing with Istio](istio-integration.md)                | `istio`                | Enables Eventing components to communicate with workloads in an Istio mesh.                                                                                                                                        | Beta, disabled by default  |

docs/eventing/features/istio-integration.md

@@ -0,0 +1,73 @@
+# Eventing integration with Istio service mesh
+
+**Flag name**: `istio`
+
+**Stage**: Beta, disabled by default
+
+**Tracking issue**: [#6596](https://github.com/knative/eventing/issues/6596)
+
+## Overview
+
+Administrators can use Istio with Eventing to encrypt, authenticate and authorize requests to
+Eventing components.
+
+## Prerequisites
+
+- In order to enable the istio integration, you will need to install Istio by
+  following [the Istio installation guides](https://istio.io/latest/docs/setup/install/).
+
+## Installation
+
+Some Eventing components use services of type `ExternalName` and with such services, Istio need to
+be manually configured to connect to such services using mutual TLS.
+
+Eventing releases a controller that automatically configures Istio so that any pod that is part of
+an Istio mesh can communicate with Eventing components that are also part of the same Istio mesh.
+
+1. Create the Eventing namespace and enable Istio injection:
+    ```shell
+    kubectl create namespace knative-eventing --dry-run=client -oyaml | kubectl apply -f -
+    kubectl label namespace knative-eventing istio-injection=enabled
+    ```
+2. [Follow Eventing installation](./../../install)
+
+3. Install `eventing-istio-controller`:
+    ```shell
+    kubectl apply -f {{ artifact(org="knative-extensions", repo="eventing-istio",file="eventing-istio.yaml")}}
+    ```
+4. Verify `eventing-istio-controller` is ready:
+    ```shell
+    kubectl get deployment -n knative-eventing
+    ```
+   Example output:
+    ```shell
+    NAME                        ...   READY
+    eventing-istio-controller   ...   True 
+    # other deployments omitted ...
+    ```
+
+## Enable istio integration
+
+The `istio` feature flag is an enum configuration that configures the `eventing-istio-controller` to
+create Istio resources for Eventing resources.
+
+The possible values for `istio` are:
+
+- `disabled`
+    - Disable Eventing integration with Istio
+- `enabled`
+    - Enabled Eventing integration with Istio
+
+For example, to enable `istio` integration, the `config-features` ConfigMap will look like
+the following:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: config-features
+  namespace: knative-eventing
+data:
+  istio: "enabled"
+```
+