Add project configuration, skeleton and Doxyfile

Add pyproject.toml (Python >=3.13, no runtime dependencies, pytest as a
dev extra), the src/higher_lower package with an empty constants module
and an entry-point stub, a smoke test, and the Doxyfile for the Doxygen
comments in the source. Ignore the Doxygen output in .gitignore.

Verified in a fresh .venv: pip install -e ".[dev]", pytest (2 passed),
python -m higher_lower and doxygen Doxyfile without warnings.

Task: MIL-001#1
Task: MIL-001#2
Task: MIL-001#3
Task: MIL-001#4
Closes #1
Closes #2
Closes #3
Closes #4

pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "higher-lower"
+version = "0.1.0"
+description = "Higher Lower console game: guess which Instagram account has more followers (Udemy 100 Days of Code, day 14)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = { file = "LICENSE" }
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]