chore(deps): update sphinx-rtd-theme requirement from <1.0 to <2.0 (#637)

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [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@0.1.8...1.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  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: Kazuaki Matsuo <fly.49.89.over@gmail.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -1,2 +1,2 @@
 sphinx >= 3.0, <4.0
-sphinx_rtd_theme < 1.0
+sphinx_rtd_theme < 2.0