chore: add git submodule action (#638)

Signed-off-by: Tao Feng <fengtao04@gmail.com>

.github/workflows/deploy_docs.yml

@@ -11,7 +11,13 @@ jobs:
     steps:
       - name: Checkout master
         uses: actions/checkout@v1
-
+      - name: Checkout submodules using a PAT
+        run: |
+          git config --file .gitmodules --get-regexp url | while read url; do
+            git config --file=.gitmodules $(echo "$url" | sed -E "s/git@github.com:|https:\/\/github.com\//https:\/\/${{ secrets.CI_PAT }}:${{ secrets.CI_PAT }}@github.com\//")
+          done
+          git submodule sync
+          git submodule update --init --recursive
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@master
         env: