docs: bump sphinx from 8.1.0 to 8.1.3 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.0 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@v8.1.0...v8.1.3)

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

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

docs/requirements.txt

@@ -1,2 +1,2 @@
 furo==2024.8.6
-sphinx==8.1.0
+sphinx==8.1.3