Add repository baseline: layout, tooling, assets, README, Doxyfile and CI (MIL-001)

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

.gitea/workflows/run-tests.yml

@@ -0,0 +1,35 @@
+# Continuous integration for the U.S. States Game.
+# Read by Gitea Actions. Installs the project with its dev extra, then checks
+# style (ruff), types (mypy) and behaviour (pytest). Design: docs/project-plan.md.
+---
+name: Run tests
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  run-tests:
+    name: Lint, type check and test
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Check out the project
+        uses: actions/checkout@v4.2.2
+      - name: Set up Python
+        uses: actions/setup-python@v5.3.0
+        with:
+          python-version: "3.13"
+      - name: Install the project and its dev tools
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e ".[dev]"
+      - name: Check style
+        run: python -m ruff check . && python -m ruff format --check .
+      - name: Check types
+        run: python -m mypy
+      - name: Check workflow files
+        run: python -m yamllint .gitea .yamllint
+      - name: Run the tests
+        run: python -m pytest