Add number guessing game project scaffold

- Add guess_the_number package with set_difficulty, check_answer and
  play_game, plus constants.py (range, attempts, ASCII logo)
- Add pytest tests for difficulty, feedback and full game flow
- Add pyproject.toml (Python >= 3.13, no runtime dependencies)
- Add docs/Doxyfile and Doxygen comments in the source
- Rewrite README with venv setup, run, test and docs instructions
- Ignore docs/build/ in .gitignore

pyproject.toml

@@ -0,0 +1,25 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "guess-the-number"
+version = "0.1.0"
+description = "Number Guessing Game - Udemy 100 Days of Code, day 12."
+readme = "README.md"
+requires-python = ">=3.13"
+license = { file = "LICENSE" }
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[project.scripts]
+guess-the-number = "guess_the_number.game:play_game"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]