CI: (deps): Bump sphinx from 8.0.2 to 8.1.3 in /ci

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.0.2 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.2...v8.1.3)

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

Signed-off-by: dependabot[bot] <[email protected]>

ci/doc_requirements.txt

@@ -1,4 +1,4 @@
-sphinx==8.0.2
+sphinx==8.1.3
 pydata-sphinx-theme==0.15.4
 sphinx-design==0.6.1
 sphinx-gallery==0.17.1