Add document to tutorial section for configuring the default oidc implementation

docs/content/tutorials/oidc-custom-configuration.md

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+---
+title: NGINX Ingress Controller and Open Service Mesh
+description: |
+  Use NGINX Ingress Controller with Open Service Mesh.
+weight: 1800
+doctypes: ["concept"]
+toc: true
+---
+
+# OIDC Custom Configuration
+
+The F5 NGINX Ingress Controller implements OpenID Connect (OIDC) using the NGINX OpenID Connect Reference implementation: [nginx-openid-connect](https://github.com/nginxinc/nginx-openid-connect).
+
+This guide will walk through how to customise and configure this default implementation.
+
+## Prerequisites
+This guide assumes that you have an F5 NGINX Ingress Controller deployed. If not, please follow the installation steps using either the [Manifest](https://docs.nginx.com/nginx-ingress-controller/installation/installation-with-manifests/) or [HELM](https://docs.nginx.com/nginx-ingress-controller/installation/installation-with-helm/) approach.
+
+To customise the NGINX OpenID Connect Reference implementation, you will need to:
+1. Create a ConfigMap containing the contents of the default `oidc.conf` file
+2. Attach a `Volume` and `VolumeMount` to your deployment of the F5 NGINX Ingress Controller
+
+This setup will allow the custom configuration in your ConfigMap to override the contents of the default `oidc.conf` file.
+
+## Step 1 - Creating the ConfigMap
+
+Run the below command to generate a ConfigMap with the contents of the `oidc.conf` file.
+**NOTE** The ConfigMap must be deployed in the same `namespace` as the F5 NGINX Ingress Controller.
+```
+kubectl create configmap oidc-config-map --from-literal=oidc.conf="$(curl -k https://raw.githubusercontent.com/nginxinc/kubernetes-ingress/v3.1.1/internal/configs/oidc/oidc.conf)"
+```
+
+Use the `kubectl describe` command to confirm the contents of the ConfigMap are correct.
+
+```
+kubectl describe configmap oidc-config-map
+```
+
+```
+Name:         oidc-config-map
+Namespace:    default
+Labels:       <none>
+Annotations:  <none>
+
+Data
+====
+oidc.conf:
+----
+    # Advanced configuration START
+    set $internal_error_message "NGINX / OpenID Connect login failure\n";
+    set $pkce_id "";
+    # resolver 8.8.8.8; # For DNS lookup of IdP endpoints;
+    subrequest_output_buffer_size 32k; # To fit a complete tokenset response
+    gunzip on; # Decompress IdP responses if necessary
+    # Advanced configuration END
+
+    ...
+    # Rest of configuration file truncated
+```
+
+## Step 2 - Customising the default configuration
+Once the contents of the `oidc.conf` file has been added to the ConfigMap, you are free to customise the contents of this ConfigMap.
+This example demonstrates adding a comment to the top of the file. This comment will be shown at the top of the `oidc.conf` file.
+This comment will be `# >> Custom Comment for my OIDC file <<`
+
+```
+kubectl edit configmap oidc-config-map
+```
+
+Add the custom content:
+```
+# Please edit the object below. Lines beginning with a '#' will be ignored,
+# and an empty file will abort the edit. If an error occurs while saving this file will be
+# reopened with the relevant failures.
+#
+apiVersion: v1
+data:
+  oidc.conf: |2-
+        # >> Custom Comment for my OIDC file <<
+        # Advanced configuration START
+        set $internal_error_message "NGINX / OpenID Connect login failure\n";
+        set $pkce_id "";
+        # resolver 8.8.8.8; # For DNS lookup of IdP endpoints;
+        subrequest_output_buffer_size 32k; # To fit a complete tokenset response
+        gunzip on; # Decompress IdP responses if necessary
+        # Advanced configuration END
+
+        ...
+        # Rest of configuration file truncated
+```
+> **IMPORTANT**
+>
+> In Step 3 an NGINX Ingress Controller will be deployed/updated that will use this ConfigMap. Any changes made to this ConfigMap must be made **before** deploying/updating the NGINX Ingress Controller. If an update is applied to the ConfigMap after the NGINX Ingress Controller is deployed, it will not get applied. Applying any updates to the data in this ConfigMap will require the NGINX Ingress Controller to be re-deployed.
+
+## Step 3 - Add Volume and VolumeMount to the Ingress Controller deployment
+
+In this step we will add a `Volume` and `VolumeMount` to the NGINX Ingress Controller deployment.
+This will allow you to mount the ConfigMap created in Step 1 and overwrite the contents of the `oidc.conf` file.
+
+This document will demonstrate how to add the `Volume` and `VolumeMount` using both Manifest and HELM
+
+### Manifest
+
+The below configuration shows where the `Volume` and `VolumeMount` can be added to your Deployment/Daemonset file.
+
+The `VolumeMount` must be added the `spec.template.spec.containers` section.
+
+The `Volume` must be added the `spec.template.spec` section:
+```
+apiVersion: apps/v1
+kind: <Deployment/Daemonset>
+metadata:
+  name: <name>
+  namespace: <ic-namespace>
+spec:
+  ...
+  ...
+  template:
+    ...
+    ...
+    spec:
+      ...
+      ...
+      volumes:
+      - name: oidc-volume
+        configMap:
+          name: <config-map-name> # Must match the name of the ConfigMap
+      containers:
+        ...
+        ...
+        volumeMounts:
+        - name: oidc-volume
+          mountPath: /etc/nginx/oidc/oidc.conf
+          subPath: oidc.conf # Must match the name in the data filed
+          readOnly: true
+```
+
+Once the `Volume` and `VolumeMount` has been added the manifest file, apply the changes to the Ingress Controller deployment.
+
+Confirm the `oidc.conf` file has been updated:
+```
+kubectl exec -it -n <ic-namespace> <ingess-controller-pod> -- cat /etc/nginx/oidc/oidc.conf
+```
+
+### Helm
+
+Deployments using helm will need to edit their existing
+Edit the NGINX Ingress Controller Deployment/Daemonset yaml to include a `Volume` and `VolumeMount`.
+
+The `Volume` should be within the `spec.template.spec` section.
+
+The `VolumeMount `must be added the `spec.template.spec.containers` section.
+
+For Deployments:
+```
+kubectl edit deployments <name-of-deployment> -n <ic-namespace>
+```
+
+For Daemonsets:
+```
+kubectl edit daemonset <name-of-daemonset> -n <ic-namespace>
+```
+
+```
+apiVersion: apps/v1
+kind: <Deployment/Daemonset>
+metadata:
+  name: <name>
+  namespace: <ic-namespace>
+spec:
+  ...
+  ...
+  template:
+    ...
+    ...
+    spec:
+      ...
+      ...
+      volumes:
+      - name: oidc-volume
+        configMap:
+          name: <config-map-name> # Must match the name of the ConfigMap
+      containers:
+        ...
+        ...
+        volumeMounts:
+        - name: oidc-volume
+          mountPath: /etc/nginx/oidc/oidc.conf
+          subPath: oidc.conf # Must match the name in the data filed
+          readOnly: true
+```
+
+Once the Deployment/Daemonset has been edited, save the file and exit.
+
+Confirm the `oidc.conf` file has been updated:
+```
+kubectl exec -it -n <ic-namespace> <ingess-controller-pod> -- cat /etc/nginx/oidc/oidc.conf
+```