chore(doc): bump sphinx in /doc in the github-actions group

Bumps the github-actions group in /doc with 1 update: [sphinx](https://github.com/sphinx-doc/sphinx).


Updates `sphinx` from 8.0.2 to 8.1.3
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.0.2...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: github-actions
...

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

Author: dependabot[bot]

doc/requirements.txt

@@ -2,5 +2,5 @@ breathe==4.35.0
 exhale==0.3.7
 myst-parser==4.0.0
 myst-parser[linkify]
-sphinx==8.0.2
+sphinx==8.1.3
 sphinx-notfound-page==1.0.4