CI: Collect coverage from docs build

.github/workflows/docs.yml

@@ -18,9 +18,18 b' jobs:'
         sudo apt-get install graphviz
     - name: Install Python dependencies
       run: |
-        python -m pip install --upgrade pip setuptools
+        python -m pip install --upgrade pip setuptools coverage
         pip install -r docs/requirements.txt
     - name: Build docs
       run: |
         python tools/fixup_whats_new_pr.py
-        make -C docs/ html SPHINXOPTS="-W"
+        make -C docs/ html SPHINXOPTS="-W" \
+          PYTHON="coverage run -a" \
+          SPHINXBUILD="coverage run -a -m sphinx.cmd.build"
+    - name: Generate coverage xml
+      run: |
+        coverage combine `find . -name .coverage\*` && coverage xml
+    - name: Upload coverage to Codecov
+      uses: codecov/codecov-action@v2
+      with:
+        name: Docs