Bump sphinx from 6.2.1 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 6.2.1 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@v6.2.1...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

@@ -1,3 +1,3 @@
-sphinx==6.2.1
+sphinx==8.1.3
 sphinx_rtd_theme
 sphinx-toolbox