Update sphinx requirement from ~=8.0 to ~=8.1

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.0...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

doc/requirements.txt

@@ -1,3 +1,3 @@
 -e .
-sphinx~=8.0
+sphinx~=8.1
 furo==2024.8.6