Run nox -s test-docstrings on GitHub actions

The xdoctest integration in pytest (see tests/test_docstrings.py)
is prone to miss docstrings in new source files as they must be
included in the test case explicitly.

So, to play it safe, we run the nox session "test-docstrings" on CI.

.github/workflows/test_docstrings.yml

@@ -0,0 +1,21 @@
+name: test-docstrings
+on: push
+jobs:
+  test-docstrings:
+    runs-on: ubuntu-latest
+    name: test-docstrings
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: 3.12
+          architecture: x64
+
+      - run: python --version
+      - run: pip --version
+
+      # The following pinned dependencies must be updated manually
+      - run: pip install nox==2024.4.15
+      - run: pip install poetry==1.8.3
+
+      - run: nox -s test-docstrings