Update sphinx requirement from ~=7.3.7 to ~=8.1.3 in /requirements

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@v7.3.7...v8.1.3)

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

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

requirements/docs.txt

@@ -1,5 +1,5 @@
 sphinx-autobuild==2024.10.3
-sphinx~=7.3.7
+sphinx~=8.1.3
 sphinx-copybutton~=0.5.2
 furo==2024.5.6
 enum-tools[sphinx]~=0.12.0