Add project setup: layout, pyproject, constants, Doxyfile, README

Set up the Blackjack project foundation for gateway MIL-001.

- pyproject.toml: Python >=3.13, src layout, no runtime dependencies,
  optional dev extras (ruff, mypy, pytest) and their configuration
- src/blackjack/: package with constants.py (deck, scoring thresholds,
  emoji card faces) and Doxygen-style docstrings
- tests/test_package.py: smoke tests for the package and its constants
- Doxyfile: reads src/, writes HTML to docs/doxygen/
- README.md: venv setup with pip upgrade, run, test, lint and Doxygen
  instructions
- .gitignore: ignore generated docs/doxygen/

The repository description and topics were set on the git host through
its API (no file change).

Closes #1
Closes #2
Closes #3
Closes #4
Closes #5
Closes #6

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Task: MIL-001#5
Task: MIL-001#6

README.md

@@ -1,2 +1,94 @@
-# 011-black-jack
+# Blackjack
 
+A console Blackjack game against a computer dealer, played with emoji cards.
+It is the day 11 capstone of Udemy's *100 Days of Code: The Complete Python Pro
+Bootcamp*, built with a plan-first process (milestones and issues in `docs/`).
+
+## House rules
+
+- The deck is unlimited; cards are not removed when drawn. No jokers.
+- Jack, Queen and King count 10. An ace counts 11 or 1.
+- A two-card hand of ace + 10 is a blackjack.
+- The computer is the dealer and draws while its score is below 17.
+
+## Requirements
+
+- Python 3.13 or newer
+- No runtime dependencies
+- Optional: [Doxygen](https://www.doxygen.nl/) to build the source documentation
+- A terminal with UTF-8 and emoji support (for example Windows Terminal)
+
+## Setup
+
+Create and activate a local virtual environment, upgrade pip, then install the
+project in editable mode with the development tools.
+
+Windows (PowerShell):
+
+```powershell
+python -m venv .venv
+.\.venv\Scripts\Activate.ps1
+python -m pip install --upgrade pip
+python -m pip install -e ".[dev]"
+```
+
+Linux and macOS:
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+python -m pip install --upgrade pip
+python -m pip install -e ".[dev]"
+```
+
+Leave the environment with `deactivate`.
+
+## Run
+
+The game entry point is added in the *Game Implementation* milestone:
+
+```bash
+python -m blackjack
+```
+
+## Test
+
+```bash
+python -m unittest discover -s tests
+```
+
+`pytest` also runs the same tests (`python -m pytest`).
+
+## Lint and type check
+
+```bash
+ruff check .
+ruff format --check .
+mypy src
+```
+
+## Documentation
+
+Generate the source documentation (Doxygen comments in `src/`) into
+`docs/doxygen/`:
+
+```bash
+doxygen Doxyfile
+```
+
+Planning documents (business case, stakeholder analysis, project plan and
+milestones) are in `docs/`.
+
+## Project layout
+
+```text
+src/blackjack/   game package (constants.py, ...)
+tests/           unit tests
+docs/            planning documents and generated Doxygen output
+pyproject.toml   project configuration
+Doxyfile         Doxygen configuration
+```
+
+## License
+
+See [LICENSE](LICENSE).