add missing entry to requirements.txt

docs/requirements.txt

@@ -2,6 +2,7 @@ docutils==0.20.1
 sphinx-copybutton
 sphinx-intl
 sphinx-rtd-theme>=0.5.1
+sphinx-rtd-dark-mode>=0.5.1
 sphinx-tabs>=3.2.0
 sphinx>=5.0.2
 sphinxext-opengraph==0.9.1