ci(workflow): add 'npm' cache for actions/setup-node in .github/workflows (#1382)

.github/workflows/generate-theme-doc.yml

@@ -14,9 +14,10 @@ jobs:
     steps:
       - uses: actions/checkout@v1
       - name: setup node
-        uses: actions/setup-node@v1
+        uses: actions/setup-node@v2
         with:
           node-version: "12.x"
+          cache: npm
 
       - name: npm install, generate readme
         run: |