Add doctests to the test suite

- use xdoctest to validate code snippets in docstrings
- make xdoctest part of the nox session "test" via
  the new `test_docstrings()` test case
- add nox session "test-docstrings" for convenience;
  also, `xdoctest.doctest_module()` does not discover
  docstrings that are imported at the root of the package
  => each new module with docstrings must be added to
     `test_docstrings()` by hand, which is likely forgotten
  => the nox session "test-docstrings" should run on CI

tests/test_docstrings.py

@@ -0,0 +1,23 @@
+"""Integrate `xdoctest` into the test suite.
+
+Ensure all code snippets in docstrings are valid and functioning code.
+
+Important: All modules with docstrings containing code snippets
+           must be put on the parameter list below by hand!
+"""
+
+import pytest
+import xdoctest
+
+
+@pytest.mark.parametrize(
+    "module",
+    [
+        "lalib",
+    ],
+)
+def test_docstrings(module):
+    """Test code snippets within the package with `xdoctest`."""
+    result = xdoctest.doctest_module(module, "all")
+
+    assert result["n_failed"] == 0