Cache pip

Based on symfony/symfony-docs#13209
by @smoench

Author: OskarStark

project/.github/workflows/documentation.yaml

@@ -30,6 +30,14 @@ jobs:
             - name: "Install Sphinx dependencies"
               run: sudo apt-get install python-dev build-essential
 
+            -   name: "Cache pip"
+                uses: actions/cache@v1
+                with:
+                    path: ~/.cache/pip
+                    key: ${{ runner.os }}-pip-${{ hashFiles('docs/requirements.txt') }}
+                    restore-keys: |
+                        ${{ runner.os }}-pip-
+
             - name: "Install custom requirements via pip"
               run: pip install -r docs/requirements.txt