Include doctests in the test suite

- use xdoctest to validate all code snippets in docstrings
- add xdoctest to the nox session "test"

src/urban_meal_delivery/__init__.py

@@ -1,4 +1,10 @@
-"""Source code for the urban-meal-delivery research project."""
+"""Source code for the urban-meal-delivery research project.
+
+Example:
+    >>> import urban_meal_delivery as umd
+    >>> umd.__version__ != '0.0.0'
+    True
+"""
 
 from importlib import metadata as _metadata