CI: don't make a shallow Git clone for docs

Author: mgeier

.github/workflows/main.yml

@@ -58,6 +58,8 @@ jobs:
       run: |
         python --version
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
     - name: Install doc dependencies
       run: |
         python -m pip install -r doc/requirements.txt