Update sphinx-rtd-theme requirement from <3,>=2 to >=2,<4 (#1900)

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

Author: dependabot[bot]

requirements/docs.txt

@@ -1,5 +1,5 @@
 sphinx>=5.3,<9
-sphinx_rtd_theme>=2,<3
+sphinx_rtd_theme>=2,<4
 readthedocs-sphinx-search~=0.3
 sphinxcontrib-shellcheck>=1,<2
 sphinx-autobuild>=2020.9.1