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