[Dependabot]: Update sphinx requirement from ~=7.4 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@v7.4.0...v8.1.0)

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

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

doc/requirements.txt

@@ -3,7 +3,7 @@
 pyTooling ~= 6.7
 
 # Enforce latest version on ReadTheDocs
-sphinx ~= 7.4
+sphinx ~= 8.1
 docutils ~= 0.20
 
 # ReadTheDocs Theme