[enh] engines: add about variable

move meta information from comment to the about variable
so the preferences, the documentation can show these information

searx/engines/dummy-offline.py

@@ -1,11 +1,19 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """
  Dummy Offline
-
- @results     one result
- @stable      yes
 """
 
 
+# about
+about = {
+    "wikidata_id": None,
+    "official_api_documentation": None,
+    "use_official_api": False,
+    "require_api_key": False,
+    "results": 'HTML',
+}
+
+
 def search(query, request_params):
     return [{
         'result': 'this is what you get',