Bump sphinx from 8.1 to 9.1.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1 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@v8.1.0...v9.1.0)

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

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

Author: dependabot[bot]

doc/requirements.txt

@@ -11,6 +11,6 @@ qiskit-aer==0.17.2
 qiskit-ibm-runtime==0.45.0
 sphinxcontrib-bibtex==2.6.5
 sphinx-automodapi==0.22.0
-sphinx==8.1
+sphinx==9.1.0
 pennylane-sphinx-theme
 git+https://github.com/PennyLaneAI/pennylane.git@main