Bump sphinx from 7.1.2 to 8.1.0 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.1.2 to 8.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.1.2...v8.1.0)

---
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==2023.3.27
-sphinx==7.1.2
+sphinx==8.1.0
 sphinx-click==4.4.0
 myst_parser==2.0.0