Bump sphinx from 9.0.4 to 9.1.0 in /docs

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] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,3 +1,3 @@
 furo==2025.12.19
-sphinx==9.0.4
+sphinx==9.1.0
 myst_parser==4.0.1