Plan project and set up Phase 1 skeleton

Add the framework submodule and the accepted planning documents:
Business Case, Stakeholder Analysis, Project Plan, three milestones,
review records RC-001 to RC-006 and the traceability matrix.

Set up the project skeleton for MIL-001: pyproject.toml (Python
>=3.13, no runtime dependencies, pytest/ruff/mypy as dev extra),
 src/coffee_machine/constants.py, a constants consistency test,
 Doxyfile and a CI workflow that runs ruff, mypy and pytest.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Closes #1
Closes #2
Closes #3
Closes #4
Closes #5

src/coffee_machine/__init__.py

@@ -0,0 +1,3 @@
+"""! @file
+@brief Object-oriented coffee machine from the 100 Days of Code course.
+"""