[docs] add engine "Demo Offline Engine"

This engine just exists for documentation purpose.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/src/searx.engines.demo_offline.rst

@@ -0,0 +1,9 @@
+.. _demo offline engine:
+
+===================
+Demo Offline Engine
+===================
+
+.. automodule:: searx.engines.demo_offline
+  :members:
+

searx/engines/demo_offline.py

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+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""Within this module we implement a *demo offline engine*.  Do not look to
+close to the implementation, its just a simple example.  To get in use of this
+*demo* engine add the following entry to your engines list in ``settings.yml``:
+
+.. code:: yaml
+
+  - name: my offline engine
+    engine: demo_offline
+    shortcut: demo
+    disabled: false
+
+"""
+
+import json
+
+engine_type = 'offline'
+categories = ['general']
+disabled = True
+timeout = 2.0
+
+about = {
+    "wikidata_id": None,
+    "official_api_documentation": None,
+    "use_official_api": False,
+    "require_api_key": False,
+    "results": 'JSON',
+}
+
+# if there is a need for globals, use a leading underline
+_my_offline_engine = None
+
+def init(engine_settings=None):
+    """Initialization of the (offline) engine.  The origin of this demo engine is a
+    simple json string which is loaded in this example while the engine is
+    initialized.
+
+    """
+    global _my_offline_engine  # pylint: disable=global-statement
+
+    _my_offline_engine = (
+        '[ {"value": "%s"}'
+        ', {"value":"first item"}'
+        ', {"value":"second item"}'
+        ', {"value":"third item"}'
+        ']'
+
+        % engine_settings.get('name')
+    )
+
+def search(query, request_params):
+    """Query (offline) engine and return results.  Assemble the list of results from
+    your local engine.  In this demo engine we ignore the 'query' term, usual
+    you would pass the 'query' term to your local engine to filter out the
+    results.
+
+    """
+    global _my_offline_engine  # pylint: disable=global-statement
+    ret_val = []
+
+    result_list = json.loads(_my_offline_engine)
+
+    for row in result_list:
+        entry = {
+            'query' : query,
+            'language' : request_params['language'],
+            'value' : row.get("value"),
+            # choose a result template or comment out to use the *default*
+            'template' : 'key-value.html',
+        }
+        ret_val.append(entry)
+
+    return ret_val