chore(deps): bump sphinx from 7.4.7 to 9.0.4 in /docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.4.7 to 9.0.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v9.0.4/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.4.7...v9.0.4)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.0.4
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,5 +1,5 @@
 shibuya==2025.8.16
-sphinx==7.4.7
+sphinx==9.0.4
 sphinx-click==6.0.0
 myst_parser==4.0.1
 sphinx-copybutton==0.5.2