fix: replace deprecated sphinx default

[dangotbanned]

Removes a warning from doc build

Before

>>> hatch run doc:build-html-win
cmd [1] | if not exist doc\_images md doc\_images
cmd [2] | sphinx-build -b html -d doc\_build\doctrees doc doc\_build\html
Running Sphinx v7.4.7
loading translations [en]... done
Converting `source_suffix = '.rst'` to `source_suffix = {'.rst': 'restructuredtext'}`.
loading pickled environment... done

After

>>> hatch run doc:build-html-win
cmd [1] | if not exist doc\_images md doc\_images
cmd [2] | sphinx-build -b html -d doc\_build\doctrees doc doc\_build\html
Running Sphinx v7.4.7
loading translations [en]... done
making output directory... done