Add to extensions also?

Mentioned in https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html#how-to-install-and-use-the-theme

docs/conf.py

@@ -85,6 +85,7 @@ def save_locust_env_variables():
     "sphinx.ext.intersphinx",
     "sphinx.ext.viewcode",
     "sphinx_search.extension",
+    "sphinx_rtd_theme",
 ]
 
 # autoclass options