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

pyproject.toml

@@ -65,6 +65,7 @@ pytest = "^8.3"
 pytest-cov = "^5.0"
 semver = "^3.0"  # to test the version identifier
 tomli = [ { python = "<3.11", version = "^2.0" } ]
+xdoctest = { extras = ["colors"], version = "^1.2" }
 
 [tool.poetry.urls]
 
@@ -264,6 +265,7 @@ module = [
     "pytest",
     "semver",
     "tomli",
+    "xdoctest",
 ]
 ignore_missing_imports = true