[Dependabot]: Update sphinx requirement from ~=8.2 to ~=9.0

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@v8.2.0...v9.0.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.0.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

doc/requirements.txt

@@ -1,7 +1,7 @@
 -r ../requirements.txt
 
 # Enforce latest version on ReadTheDocs
-sphinx ~= 8.2
+sphinx ~= 9.0
 docutils ~= 0.21
 docutils_stubs ~= 0.0.22