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

Doxyfile

@@ -0,0 +1,14 @@
+# Doxygen configuration for the coffee machine. Run: doxygen Doxyfile
+PROJECT_NAME           = "Coffee Machine"
+PROJECT_BRIEF          = "Day 15 of Udemy's 100 Days of Code Python bootcamp"
+OUTPUT_DIRECTORY       = docs/doxygen
+INPUT                  = src
+FILE_PATTERNS          = *.py
+RECURSIVE              = YES
+OPTIMIZE_OUTPUT_JAVA   = YES
+EXTRACT_ALL            = YES
+JAVADOC_AUTOBRIEF      = YES
+GENERATE_HTML          = YES
+GENERATE_LATEX         = NO
+QUIET                  = YES
+WARN_IF_UNDOCUMENTED   = YES