docs(helm): add example of public oci chart (argoproj#17000)

There doesn't appear to be an example of using an OCI helm chart repository, so this adds a simple declarative example. This is a common question from the community.

Signed-off-by: Nicholas Morey <nicholas@morey.tech>

docs/user-guide/helm.md

@@ -25,6 +25,23 @@ spec:
     namespace: kubeseal
 ```
 
+Another example using a public OCI helm chart:
+```
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: nginx
+spec:
+  project: default
+  source:
+    chart: nginx
+    repoURL: registry-1.docker.io/bitnamicharts  # note: the oci:// syntax is not included.
+    targetRevision: 15.9.0
+  destination:
+    name: "in-cluster"
+    namespace: nginx
+```
+
 !!! note "When using multiple ways to provide values"
     Order of precedence is `parameters > valuesObject > values > valueFiles > helm repository values.yaml` (see [Here](./helm.md#helm-value-precedence) for a more detailed example)