Bump sphinx from 7.3.7 to 8.1.0 in /{{cookiecutter.project_slug}}/docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.3.7 to 8.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.3.7...v8.1.0)

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

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

{{cookiecutter.project_slug}}/docs/requirements.txt

@@ -5,7 +5,7 @@ emoji==2.11.1
 importlib-metadata==7.1.0
 myst-parser==2.0.0
 pygments==2.18.0
-sphinx==7.3.7
+sphinx==8.1.0
 sphinx-autoapi==3.0.0
 sphinx-rtd-theme==2.0.0
 sphinxcontrib-confluencebuilder==2.5.1