[Dependabot]: 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...v8.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

doc/requirements.txt

@@ -6,7 +6,7 @@ ruamel.yaml ~= 0.18.6
 setuptools ~= 75.1
 
 # Enforce latest version on ReadTheDocs
-sphinx ~= 8.0
+sphinx ~= 8.1
 docutils ~= 0.21
 
 # ReadTheDocs Theme