Add CI to run rethemendex.sh (#9577)

.github/workflows/static_analysis.yaml

@@ -76,6 +76,16 @@ jobs:
     name: "i18n Check"
     uses: matrix-org/matrix-react-sdk/.github/workflows/i18n_check.yml@develop
 
+  rethemendex_lint:
+    name: "Rethemendex Check"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      
+      - run: ./res/css/rethemendex.sh
+      
+      - run: git diff --exit-code
+
   js_lint:
     name: "ESLint"
     runs-on: ubuntu-latest