ci(gh-pages): Bump Python version to 3.12 for gh-pages workflow

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[ns-rse]

Python version isn't the source of the error but I think I have narrowed it down to the docutils version since we see...

  Extension error (sphinx_autodoc_typehints):
  Handler <function process_docstring at 0x7efece500fe0> for event 'autodoc-process-docstring' threw an exception (exception: The frontend.OptionParser class will be replaced by a subclass of argparse.ArgumentParser in Docutils 0.21 or later.)
  Traceback (most recent call last):
    File "/opt/hostedtoolcache/Python/3.12.7/x64/bin/sphinx-multiversion", line 8, in <module>
      sys.exit(main())
               ^^^^^^
    File "/opt/hostedtoolcache/Python/3.12.7/x64/lib/python3.12/site-packages/sphinx_multiversion/main.py", line 338, in main
      subprocess.check_call(cmd, cwd=current_cwd)
    File "/opt/hostedtoolcache/Python/3.12.7/x64/lib/python3.12/subprocess.py", line 413, in check_call
      raise CalledProcessError(retcode, cmd)
  subprocess.CalledProcessError: Command '('/opt/hostedtoolcache/Python/3.12.7/x64/bin/python', '-R', '-m', 'sphinx', '-D', 'smv_metadata_path=/tmp/tmpqlj8qchx/versions.json', '-D', 'smv_current_version=v2.0.0', '-c', '/home/runner/work/TopoStats/TopoStats/docs', '/tmp/tmpqlj8qchx/d819d1f3d0bed977cdfa926fb309a76a8e394462/docs', '/home/runner/work/TopoStats/TopoStats/docs/_build/html/v2.0.0')' returned non-zero exit status 2.

EDIT : And on searching for a solution I find I encountered this same error back in May 2024 but clearly didn't have time to investigate and fix it so am getting bitten a second time!