Docs: build with Sphinx 5 (#70309)
Fixes #60979. Also see #61045 and https://github.com/sphinx-doc/sphinx/issues/9395 for discussion.
I _believe_ the reason that we were previously pinning to Sphinx 3 was because of issues with pytorch_sphinx_theme and Sphinx 4 support, but these seem to have been resolved now. See https://torchgeo.readthedocs.io/ for an example of docs built with pytorch_sphinx_theme and Sphinx 4.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70309
Approved by: https://github.com/albanD
