ci: Add cache step

.github/workflows/docs.yml

@@ -16,6 +16,15 @@ jobs:
       - name: Set up Python 3.x
         uses: actions/setup-python@v2
 
+      - name: Cache dependencies
+        uses: actions/cache@v2
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ hashFiles('requirements.txt') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-
+            ${{ runner.os }}-
+
       - name: Install dependencies
         run: cd docs && make install