require sphinx<8.2.0 (#1119)

Contributes to rapidsai/build-planning#155

Fixes docs builds by temporarily putting a ceiling on `sphinx`.

## Notes for Reviewers

In case you're wondering why you don't see `dependencies.yaml` changes... this project uses readthedocs to builds its docs.

* builds: https://app.readthedocs.org/projects/ucx-py/builds/
* config: https://github.com/rapidsai/ucx-py/blob/674e731c21e3a85045239a943248b23dab019619/.readthedocs.yml#L16-L17

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #1119

conda/environments/builddocs.yml

@@ -4,8 +4,8 @@ channels:
 - nvidia
 - conda-forge
 dependencies:
-# required for building docs
-- sphinx
+# the ceiling on sphinx can be removed when https://github.com/spatialaudio/nbsphinx/issues/825 is resolved
+- sphinx>=8.0,<8.2.0
 - sphinx-markdown-tables
 - sphinx_rtd_theme
 - sphinxcontrib-websupport