Bump sphinx from 5.3.0 to 6.1.2 in /requirements

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 5.3.0 to 6.1.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v5.3.0...v6.1.2)

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

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

Author: dependabot[bot]

requirements/requirements_docs.txt

@@ -6,7 +6,7 @@ nbsphinx==0.8.11
 pypandoc==1.10
 pytest-sphinx==0.5.0
 pyvista==0.36.1
-sphinx==5.3.0
+sphinx==6.1.2
 sphinx-autobuild==2021.3.14
 sphinx-copybutton==0.5.0
 sphinx-gallery==0.11.0