fix: add node setup step

hugo modules still need node + autoprefixer + postcss

https://www.docsy.dev/docs/get-started/docsy-as-module/installation-prerequisites/#install-postcss

Author: alee

.github/workflows/gh-pages.yml

@@ -20,6 +20,13 @@ jobs:
           . ./.env
           echo "::set-output name=HUGO_VERSION::${HUGO_VERSION}"
 
+      - name: Setup Yarn
+        uses: actions/setup-node@v3
+        with:
+          node-version: '16'
+        run: |
+          yarn add -D postcss autoprefixer postcss-cli
+
       - name: Setup Hugo 
         uses: peaceiris/actions-hugo@v2
         with: