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]>

docs/doc_requires.txt

@@ -1,4 +1,4 @@
-sphinx~=7.4
+sphinx~=8.1
 nbsphinx~=0.9
 pygments~=2.18
 shibuya~=2024.7