Add the continuous integration workflow

Installs the project, then runs ruff, mypy, pytest and the Doxygen build. No step
opens a turtle window. GitHub Actions and Gitea Actions both read this folder.

Task: MIL-001#5

.github/workflows/ci.yml

@@ -0,0 +1,44 @@
+name: CI
+
+# Read by GitHub Actions and by Gitea Actions. No step opens a turtle window:
+# the tests use a recording fake pen.
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  checks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python 3.13
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.13"
+
+      - name: Install Doxygen
+        run: sudo apt-get update && sudo apt-get install --yes doxygen
+
+      - name: Install the project and its development tools
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install -e ".[dev]"
+
+      - name: Lint
+        run: ruff check .
+
+      - name: Check formatting
+        run: ruff format --check .
+
+      - name: Check types
+        run: mypy
+
+      - name: Run the tests
+        run: python -m pytest
+
+      - name: Build the source documentation
+        run: doxygen Doxyfile