Bump sphinx from 9.0.4 to 9.1.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 9.0.4 to 9.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v9.0.4...v9.1.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.1.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

doc/requirements.txt

@@ -1,7 +1,7 @@
 click==8.3.1
 entrypoints==0.4
 Pillow==12.1.0
-Sphinx==9.0.4
+Sphinx==9.1.0
 furo==2025.12.19
 ipython==9.9.0
 netCDF4==1.7.4