github action added to build sphinx-based doc.

.github/workflows/documentation.yaml

@@ -15,6 +15,7 @@ jobs:
           export BME_PROJECT_DIR="${GITHUB_WORKSPACE}"
           echo "Python version: "`python --version`
           echo "pip version: "`pip --version`
+          pip install Sphinx==5.3.0
           #python -m venv "${GITHUB_WORKSPACE}/.virtualenv" \
           #&& pip install --requirement "${GITHUB_WORKSPACE}/python-virtualenvs/multigit-development.requirements"
       - name: Sphinx build