Bump sphinx from 7.3.7 to 8.1.3 in /docs

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]>

docs/requirements.txt

@@ -1,2 +1,2 @@
-sphinx==7.4.7
+sphinx==8.1.3
 recommonmark==0.7.1