Add workflow to render all helm charts and verify it does not cause a git diff

.forgejo/workflows/validate-helm-render.yaml

@@ -0,0 +1,14 @@
+on:
+  push:
+    paths:
+      - helm/*
+jobs:
+  render-helm:
+    runs-on: docker
+    container:
+      image: library/alpine
+    steps:
+      - run: apk add --no-cache nodejs git helm kubectl
+      - uses: actions/checkout@v5
+      - run: helm/render-all.sh
+      - run: git diff --exit-code