Bump sphinx from 7.2.6 to 8.1.3 in /backend/docs

Bumps [sphinx](/~https://github.com/sphinx-doc/sphinx) from 7.2.6 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@v7.2.6...v8.1.3)

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

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

backend/docs/requirements.txt

@@ -1,5 +1,5 @@
 myst_parser==2.0.0
 nbsphinx==0.9.6
-sphinx==7.2.6
+sphinx==8.1.3
 sphinx-copybutton
 sphinx-rtd-theme==2.0.0