Modernize the docs publishing workflow

[ubyndr]

• Updated .github/workflows/publish-docs.yml to use the latest actions/checkout@v4/setup-python@v4, bump the build to Python 3.11, and install Poetry via snok/install-poetry@v1 with in-project virtualenvs.
• Added caching for the .venv folder keyed on poetry.lock so doc builds are faster, and split the poetry install step so we skip it on cache hits.
• Replaced the cd docs && make html shell block with poetry run sphinx-build -b html docs docs/_build/html so the workflow mirrors our local build command.