Bump sphinx-rtd-theme from 1.0.0 to 1.2.0 (#1869)

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 1.0.0 to 1.2.0.
- [Release notes](https://github.com/readthedocs/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.0.0...1.2.0)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -1,5 +1,5 @@
 sphinx==1.8.6
-sphinx-rtd-theme==1.0.0
+sphinx-rtd-theme==1.2.0
 sphinxcontrib-napoleon==0.7
 invoke==1.7.1
 jinja2==2.11.3