Update sphinx-rtd-theme requirement from <3,>=2 to >=2,<4

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@2.0.0...3.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/docs.txt

@@ -1,5 +1,5 @@
 sphinx>=5.3,<8
-sphinx_rtd_theme>=2,<3
+sphinx_rtd_theme>=2,<4
 readthedocs-sphinx-search~=0.3
 sphinxcontrib-shellcheck>=1,<2
 sphinx-autobuild>=2020.9.1