Add a GitHub action for periodically running vale

.github/workflows/periodic-style-checks.yml

@@ -0,0 +1,20 @@
+name: Periodic Style Checks
+
+on:
+  schedule:
+    - cron: "0 1 * * 4" # Runs at 01:00 AM on every Wednesday
+
+jobs:
+  vale:
+    name: Style checker
+    runs-on: ubuntu-22.04
+    defaults:
+        run:
+            shell: bash
+            working-directory: 'sp-docs'
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run vale
+        run: |
+          make vale
+