Set up project skeleton and tooling for MIL-001

Add pyproject.toml (Python >=3.13, no runtime dependencies, pytest, ruff
and mypy as dev extras), a Doxyfile that scans src/, the
src/coffee_machine package with stub constants and main modules, and a
pytest smoke test. Ignore Doxygen output in .gitignore.

Closes #1
Closes #2
Closes #3
Closes #4
Refs #5

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4

tests/test_smoke.py

@@ -0,0 +1,13 @@
+"""!
+@file test_smoke.py
+@brief Smoke test that the package imports.
+"""
+
+from coffee_machine import main as main_module
+
+
+def test_main_is_callable() -> None:
+    """!
+    @brief The entry point runs without error.
+    """
+    main_module.main()