[Dependabot]: Update sphinx-rtd-theme requirement from ~=3.0 to ~=3.1

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@3.0.0...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>

Author: dependabot[bot]

doc/requirements.txt

@@ -6,7 +6,7 @@ docutils ~= 0.22
 docutils_stubs ~= 0.0.22
 
 # ReadTheDocs Theme
-sphinx_rtd_theme ~= 3.0
+sphinx_rtd_theme ~= 3.1
 
 # Sphinx Extenstions
 sphinxcontrib-mermaid ~= 1.2