Add guide for openshift webhook

As we now have working triggers for openshift webhooks, let's document it.

Signed-off-by: Michal Konecny <mkonecny@redhat.com>

modules/ROOT/pages/webhook_openshift.adoc

@@ -0,0 +1,93 @@
+= Webhook for build trigger in Fedora OpenShift
+
+== Purpose
+
+This document explains how to setup an automatic build trigger for OpenShift deployment from
+Forgejo instance.
+
+== Setting up the OpenShift trigger
+
+. Request a new secret for your project from Fedora Infrastructure. Open a ticket on
+  https://forge.fedoraproject.org/infra/tickets/issues and request a secret
+  for your OpenShift project for both staging and/or production.
+
+. Add this secret to secret file in your OpenShift project. The secret file could look like this:
++
+....
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: generic-webhook-secret
+data:
+  WebHookSecretKey: "{{ (env == 'production')|ternary(<project>_prod_generic_secret, <project>_stg_generic_secret) | b64encode }}"
+type: Opaque
+....
+
+. Add generic build trigger to `buildconfig` file.
++
+....
+  triggers:
+    - type: Generic
+      generic:
+        secretReference:
+          name: generic-webhook-secret
+....
+
+== Obtain webhook URL
+
+. Install the OpenShift client from OKD tools. See
+  link:https://docs.okd.io/latest/cli_reference/openshift_cli/getting-started-cli.html#cli-getting-started[Kubernetes documentation].
+
+. Login to OpenShift instance you want to work with (
+  link:https://console-openshift-console.apps.ocp.fedoraproject.org/[production] or
+  link:https://console-openshift-console.apps.ocp.stg.fedoraproject.org/[staging]) and copy the
+  login command (you can find it under your profile in top right corner)
+
+. Login using the `oc` tool.
++
+....
+oc login --token=XXX --server=https://api.ocp.[stg.]fedoraproject.org:6443
+....
+
+. Switch to your project
++
+....
+oc project <project>
+....
+
+. Get the webhook url from the buildconfig
++
+....
+oc describe bc
+....
++
+And look for the Webhook Generic section, where you will find the URL.
+
+. Obtain the secret to fill in to URL.
++
+....
+oc get secret generic-webhook-secret -o jsonpath='{.data.WebHookSecretKey}' | base64 -d
+....
+
+Now you should have everything to construct the URL for webhook.
+
+== Add webhook to Forgejo project
+
+. Go to project `Settings` and `Webhooks`
+
+. Add webhook and choose Forgejo
+
+. Fill in the webhook
++
+....
+Target URL: https://api.ocp.fedoraproject.org:6443/apis/build.openshift.io/v1/namespaces/poddlers/buildconfigs/toddlers/webhooks/<secret>/generic
+HTTP method: POST
+Post content type: application/json
+Secret: 
+Trigger on: Push events
+Branch filter: <branch you want to trigger the build from>
+Authorization header:
+....
+
+Now the trigger should work everytime you push in the selected branch.