Fix docs requirements in ci

Author: wu-clan

.github/workflows/docs.yml

@@ -26,7 +26,7 @@ jobs:
           restore-keys: |
             mkdocs-material-
       - run: sudo apt-get install pngquant
-      - run: pip install git+https://${GH_TOKEN}@github.com/squidfunk/mkdocs-material.git
+      - run: pip install -r $GITHUB_WORKSPACE/requirements_docs.txt
       - run: mkdocs gh-deploy --force
 env:
   GH_TOKEN: ${{ secrets.GH_TOKEN }}