Bump sphinx-rtd-theme from 3.0.2 to 3.1.0 in /client

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 3.0.2 to 3.1.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@3.0.2...3.1.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-version: 3.1.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

client/pyproject.toml

@@ -35,7 +35,7 @@ certgrinder = "certgrinder.certgrinder:main"
 [project.optional-dependencies]
 dev = ["twine == 6.1.0", "pre-commit == 4.2.0", "setuptools-scm == 8.3.1", "build==1.2.2.post1"]
 test = ["pytest == 8.3.5", "pytest-cov==6.1.1", "tox == 4.26.0"]
-docs = ["Sphinx==8.2.3", "sphinx-rtd-theme==3.0.2", "sphinx-argparse==0.4.0"]
+docs = ["Sphinx==8.2.3", "sphinx-rtd-theme==3.1.0", "sphinx-argparse==0.4.0"]
 
 [project.urls]
 homepage = "https://github.com/tykling/certgrinder"