Update sphinx-rtd-theme requirement from <1.1 to <2.1

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@0.1.8...2.0.0)

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

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

requirements/docs.txt

@@ -1,3 +1,3 @@
 sphinx <3
 Jinja2 <3.1
-sphinx-rtd-theme <1.1
+sphinx-rtd-theme <2.1