chore(deps): bump sphinx from 7.2.6 to 8.1.3 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.2.6 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.2.6...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

@@ -1,4 +1,4 @@
 furo==2023.9.10
-sphinx==7.2.6
+sphinx==8.1.3
 sphinx-click==5.1.0
 myst_parser==2.0.0