Revert Python Version to 3.9 for Documentation Build Workflow

[fatwir]

Summary

This PR reverts the Python version used in the GitHub Actions workflow for building the project documentation from Python 3.12 back to 3.9. The change addresses the build failures encountered when using Python 3.12, ensuring that the documentation builds successfully as it did previously. An attempt was made to update the GitHub Actions workflow to use Python 3.12 for building the documentation. However, this resulted in the following error:

Extension error:
Could not import extension m2r2 (exception: No module named 'pkg_resources')

This error indicates compatibility issues between the documentation build process and Python 3.12, likely related to dependencies that are not yet fully compatible with the latest Python version.