Bump sphinx from 8.0.2 to 8.1.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.0.2 to 8.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.2...v8.1.0)

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

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

docs/requirements.txt

@@ -1,7 +1,7 @@
 numpy>=1.22.2
 matplotlib
 scipy
-Sphinx==8.0.2
+Sphinx==8.1.0
 sphinx-argparse==0.5.2
 sphinx-copybutton==0.5.2
 meshio==5.3.5