[dependabot] Bump sphinx-rtd-theme from 1.1.1 to 2.0.0 in /docs

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 1.1.1 to 2.0.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.1.1...2.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

docs/requirements.txt

@@ -1,7 +1,7 @@
 Sphinx==4.5.0
 sphinx-tabs==3.3.1
 recommonmark==0.7.1
-sphinx-rtd-theme==1.1.1
+sphinx-rtd-theme==2.0.0
 docutils<0.18
 jinja2==3.1.4
 sphinx-copybutton==0.5.2