Add project scaffold: pyproject, Doxyfile and package layout

MIL-001 Project scaffold.

- Add pyproject.toml: Python 3.13 or newer, no runtime dependencies,

  pytest as the dev extra

- Add src/calc/ package and tests/ folder

- Add Doxyfile writing API docs to docs/doxygen/

- Ignore docs/doxygen/ in .gitignore

Closes #15

Closes #16

Closes #17

Closes #18

Task: MIL-001#1

Task: MIL-001#2

Task: MIL-001#3

Task: MIL-001#4

pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["setuptools>=69"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "calc"
+version = "0.1.0"
+description = "A console calculator (Udemy 100 Days of Code, day 10)."
+readme = "README.md"
+requires-python = ">=3.13"
+license = { text = "AGPL-3.0-or-later" }
+dependencies = []
+
+[project.optional-dependencies]
+dev = ["pytest>=8"]
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+pythonpath = ["src"]