Add section "Webhook" in "Troubleshooting"

Fixes kanisterio#1292

docs/troubleshooting.rst

@@ -48,3 +48,15 @@ or file an issue on `GitHub
 <https://github.com/kanisterio/kanister/issues>`_. A `mailing list
 <https://groups.google.com/forum/#!forum/kanisterio>`_ is also
 available if needed.
+
+Webhook
+=======
+For the webhook to work, the Kubernetes API needs to connect to port 9443 of the operator. For some clusters, a firewall has to be configured.
+
+GKE
+---
+If you get an error, that the webhook can't be reached you need to check if your firewall misses a rule for port 9443::
+
+  Error from server (InternalError): error when creating "blueprint.yaml": Internal error occurred: failed calling webhook "blueprints.cr.kanister.io": failed to call webhook: Post "https://kanister-kanister-operator.kanister.svc:443/validate/v1alpha1/blueprint?timeout=5s": context deadline exceeded
+
+See https://cloud.google.com/kubernetes-engine/docs/how-to/private-clusters#add_firewall_rules and https://github.com/kubernetes/kubernetes/issues/79739