docs(vercel): add missing steps required on the action

.github/workflows/preview.yml

@@ -6,15 +6,34 @@ on:
   push:
     branches-ignore:
       - main
+
 jobs:
   Deploy-Preview:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
         with:
+          token: ${{ secrets.VTEX_GITHUB_BOT_TOKEN }}
           fetch-depth: 0
 
+      - name: Setup pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9.4.0
+
+      - name: Setup Node.js environment
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: "pnpm"
+
+      - name: Install dependencies
+        run: pnpm install
+
+      - name: Build
+        run: pnpm build
+
       - name: Pull Vercel Environment Information
         run: npx vercel pull --yes --environment=preview --token=${{ secrets.VERCEL_TOKEN }}