- 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
