Update sphinx-rtd-theme requirement from ~=0.5.0 to ~=2.0.0

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](readthedocs/sphinx_rtd_theme@0.5.0...2.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -1,6 +1,6 @@
 Sphinx~=3.3.1
 sphinx-automodapi~=0.13
-sphinx_rtd_theme~=0.5.0
+sphinx_rtd_theme~=2.0.0
 sphinxcontrib-napoleon~=0.7
 sphinx-click~=2.5.0
 sphinxcontrib-httpdomain~=1.7.0