Generate editor docs on a thread

* The main generation could not be moved to a thread, as it instantiates
  classes to get default values, interacts with ProjectSettings, etc.
* Only uncompressing documentation and merging it is threaded.
* Seems to improve editor load times by 0.5 seconds.

editor/editor_node.cpp

@@ -7211,7 +7211,7 @@ EditorNode::~EditorNode() {
 	EditorTranslationParser::get_singleton()->clean_parsers();
 
 	remove_print_handler(&print_handler);
-	memdelete(EditorHelp::get_doc_data());
+	EditorHelp::cleanup_doc();
 	memdelete(editor_selection);
 	memdelete(editor_plugins_over);
 	memdelete(editor_plugins_force_over);