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

Doxyfile

@@ -0,0 +1,17 @@
+# Doxygen configuration for the coffee machine (Python source with Doxygen comments).
+# Build with: doxygen Doxyfile   (output goes to docs/doxygen/, which is ignored by git)
+PROJECT_NAME           = "OOP Coffee Machine"
+PROJECT_BRIEF          = "Udemy 100 Days of Code: object-oriented coffee machine"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src README.md
+USE_MDFILE_AS_MAINPAGE = README.md
+FILE_PATTERNS          = *.py *.md
+RECURSIVE              = YES
+OPTIMIZE_OUTPUT_JAVA   = YES
+EXTRACT_ALL            = YES
+EXTRACT_PRIVATE        = NO
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES
+WARN_IF_UNDOCUMENTED   = YES
+WARN_AS_ERROR          = NO