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

src/guess_the_number/__init__.py

@@ -0,0 +1,6 @@
+## @file __init__.py
+#  @brief Number Guessing Game package.
+
+from guess_the_number.game import check_answer, play_game, set_difficulty
+
+__all__ = ["check_answer", "play_game", "set_difficulty"]