Update ci.yml

.github/workflows/ci.yml

@@ -15,7 +15,9 @@ jobs:
       - uses: actions/setup-python@v4
         with:
           python-version: 3.x
-      # - run: sudo apt install doxygen
-      - run: pip install -r requirements.txt
+      - uses: actions/cache@v2
+        with:
+          key: ${{ github.ref }}
+          path: .cache
       - run: pip install mkdocs-material
       - run: mkdocs gh-deploy --force