Update sphinx-rtd-theme requirement (#78)

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](readthedocs/sphinx_rtd_theme@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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Ryan Hill <ryanjh88@gmail.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,5 +1,5 @@
 sphinx>=7.4,<9.1
 sphinx-autodoc-typehints>=1.24,<3.7
-sphinx-rtd-theme>=2.0.0,<3.0.3
+sphinx-rtd-theme>=2.0.0,<3.1.1
 docutils<0.23
 sphinx-copybutton