Fix CI: Temporarily ignore deprecation warnings from Sphinx

[hugovk]

Re: #68 (comment)

Sphinx is failing on the CI because we're turning warnings into errors (-W error) and Sphinx is using imghdr, and triggering a deprecation warning:

Run python -bb -X dev -W error -m sphinx --color -n -E -a -W --keep-going docs build
Running Sphinx v5.3.0

Exception occurred:
  File "/opt/hostedtoolcache/Python/3.11.0/x64/lib/python3.11/warnings.py", line 514, in _deprecated
    warn(msg, DeprecationWarning, stacklevel=3)
DeprecationWarning: 'imghdr' is deprecated and slated for removal in Python 3.13
The full traceback has been saved in /tmp/sphinx-err-tfj_[8](https://github.com/hugovk/docs-community/actions/runs/3445416680/jobs/5749102789#step:5:9)37y.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

Let's temporarily disable deprecation warnings until sphinx-doc/sphinx#10440 is fixed.

[CAM-Gerlach]

LGTM, thanks @hugovk

[encukou]

Thanks for getting to this!