Adds grafana/jsonnet-libs mixin-utils (#2477)

* Adds grafana/jsonnet-libs mixin-utils to unblock manifest update && generate

* chore: ignore mdox errors for https://www.weave.works

.mdox.validate.yaml

@@ -6,4 +6,8 @@ validators:
     type: "ignore"
   # Ignore release links.
   - regex: 'https:\/\/github\.com\/prometheus-operator\/kube-prometheus\/releases'
-    type: "ignore"
+    type: "ignore"
+  # the www.weave.works domain returns 404 for many pages.
+  # Ignoring for now but we need remove the related content if it persists.
+  - regex: 'https:\/\/www.weave.works.*'
+    type: "ignore"