fix(deps): bump sphinx from 7.3.7 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.3.7 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@v7.3.7...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]>

Author: dependabot[bot]

requirements-docs.txt

@@ -1,5 +1,5 @@
 matplotlib==3.9.0
 numpydoc==1.8.0
-sphinx==7.3.7
+sphinx==8.1.3
 sphinx-gallery==0.18.0
 sphinx-rtd-theme==3.0.2