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

src/lalib/__init__.py

@@ -1,4 +1,10 @@
-"""A Python library to study linear algebra."""
+"""A Python library to study linear algebra.
+
+First, verify that your installation of `lalib` works:
+>>> import lalib
+>>> lalib.__version__ != '0.0.0'
+True
+"""
 
 from importlib import metadata