Add Vale workflow

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

.github/workflows/vale.yml

@@ -0,0 +1,21 @@
+name: Lint prose with Vale
+on:
+  pull_request:
+    paths: ["docs/sources/**"]
+  workflow_dispatch:
+jobs:
+  vale:
+    runs-on: ubuntu-latest
+    container:
+      image: grafana/vale:latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Run linter
+        run: >
+          /bin/reviewdog
+          --conf=/etc/vale/.reviewdog.yml
+          --fail-on-error
+          --reporter=github-pr-review
+        env:
+          REVIEWDOG_GITHUB_API_TOKEN: ${{ secrets.GITHUB_TOKEN }}