Update sphinx-rtd-theme requirement from >=2.0.0 to >=3.1.0

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](https://github.com/readthedocs/sphinx_rtd_theme/compare/2.0.0...3.1.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-version: 3.1.0
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -1,6 +1,6 @@
 # Documentation requirements for Sphinx
 sphinx>=7.1.0
-sphinx-rtd-theme>=2.0.0
+sphinx-rtd-theme>=3.1.0
 myst-parser>=3.0.0
 sphinxcontrib-napoleon>=0.7
 linkify-it-py>=2.0.0