Fix broken Read the Docs build: install sphinx_rtd_theme - #23

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v2, actions/setup-python@v5, codecov/codecov-action@v3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/