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

README.md

@@ -1,2 +1,77 @@
-# 012-guess_the_number
+# Guess The Number
 
+A terminal Number Guessing Game, built as the day 12 project of Udemy's
+*100 Days of Code™: The Complete Python Pro Bootcamp*.
+
+The computer picks a number between 1 and 100. Choose a difficulty and guess it:
+
+| Level | Attempts |
+|-------|----------|
+| easy  | 10       |
+| hard  | 5        |
+
+After each guess you are told whether it was too high or too low.
+
+## Requirements
+
+- Python 3.13 or newer
+- No runtime dependencies (`pytest` is only needed for the tests)
+
+## Setup
+
+Create and activate a local virtual environment, then upgrade pip.
+
+Windows (PowerShell):
+
+```powershell
+python -m venv .venv
+.venv\Scripts\Activate.ps1
+python -m pip install --upgrade pip
+```
+
+Linux / macOS:
+
+```bash
+python -m venv .venv
+source .venv/bin/activate
+python -m pip install --upgrade pip
+```
+
+## Run the game
+
+```bash
+python -m pip install -e .
+python -m guess_the_number
+```
+
+## Run the tests
+
+```bash
+python -m pip install -e ".[dev]"
+python -m pytest
+```
+
+## Documentation
+
+Source comments use Doxygen. With [Doxygen](https://www.doxygen.nl/) installed:
+
+```bash
+doxygen docs/Doxyfile
+```
+
+The HTML output is written to `docs/build/html`.
+
+## Project layout
+
+```
+src/guess_the_number/   game code (constants.py, game.py)
+tests/                  pytest tests
+docs/                   Doxyfile
+```
+
+The functions `set_difficulty`, `check_answer` and `play_game` follow the
+course assignment's naming.
+
+## License
+
+See [LICENSE](LICENSE).