chore: bump sphinx from 7.2.6 to 8.1.3

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/v8.1.3/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] <support@github.com>

docs/requirements.txt

@@ -1 +1 @@
-Sphinx==7.2.6
+Sphinx==8.1.3