Bump sphinx from 7.1.0 to 8.1.3 in /requirements

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.1.0 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.1.0...v8.1.3)

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

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

requirements/requirements_docs.txt

@@ -7,7 +7,7 @@ nbsphinx==0.9.5
 pypandoc==1.14
 pytest-sphinx==0.6.3
 pyvista==0.44.1
-sphinx==7.1.0
+sphinx==8.1.3
 sphinx-copybutton==0.5.2
 sphinx-gallery==0.18.0
 sphinx-notfound-page==1.0.4