Enable sphinx_search.extension added in #3277

I forgot to push this change in the previous PR...

conf.py

@@ -14,6 +14,7 @@ needs_sphinx = "1.3"
 sys.path.append(os.path.abspath("_extensions"))
 extensions = [
     "gdscript",
+    "sphinx_search.extension",
     "sphinx_tabs.tabs",
     "sphinx.ext.imgmath",
 ]