[docs] add engine "Demo Online Engine"

This engine just exists for documentation purpose.

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

docs/src/searx.engines.demo_online.rst

@@ -0,0 +1,9 @@
+.. _demo online engine:
+
+==================
+Demo Online Engine
+==================
+
+.. automodule:: searx.engines.demo_online
+  :members:
+

searx/engines/demo_online.py

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+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+"""Within this module we implement a *demo online engine*.  Do not look to
+close to the implementation, its just a simple example which queries `The Art
+Institute of Chicago <https://www.artic.edu>`_
+
+To get in use of this *demo* engine add the following entry to your engines
+list in ``settings.yml``:
+
+.. code:: yaml
+
+  - name: my online engine
+    engine: demo_online
+    shortcut: demo
+    disabled: false
+
+"""
+
+from json import loads
+from urllib.parse import urlencode
+
+engine_type = 'offline'
+categories = ['general']
+disabled = True
+timeout = 2.0
+categories = ['images']
+paging = True
+page_size = 20
+
+search_api = 'https://api.artic.edu/api/v1/artworks/search?'
+image_api = 'https://www.artic.edu/iiif/2/'
+
+about = {
+    "website": 'https://www.artic.edu',
+    "wikidata_id": 'Q239303',
+    "official_api_documentation": 'http://api.artic.edu/docs/',
+    "use_official_api": True,
+    "require_api_key": False,
+    "results": 'JSON',
+}
+
+
+# if there is a need for globals, use a leading underline
+_my_online_engine = None
+
+def init(engine_settings):
+    """Initialization of the (online) engine.  If no initialization is needed, drop
+    this init function.
+
+    """
+    global _my_online_engine  # pylint: disable=global-statement
+    _my_online_engine = engine_settings.get('name')
+
+def request(query, params):
+    """Build up the ``params`` for the online request.  In this example we build a
+    URL to fetch images from `artic.edu <https://artic.edu>`__
+
+    """
+    args = urlencode({
+        'q' : query,
+        'page' : params['pageno'],
+        'fields' : 'id,title,artist_display,medium_display,image_id,date_display,dimensions,artist_titles',
+        'limit' : page_size,
+        })
+    params['url'] = search_api + args
+    return params
+
+def response(resp):
+    """Parse out the result items from the response.  In this example we parse the
+    response from `api.artic.edu <https://artic.edu>`__ and filter out all
+    images.
+
+    """
+    results = []
+    json_data = loads(resp.text)
+
+    for result in json_data['data']:
+
+        if not result['image_id']:
+            continue
+
+        results.append({
+            'url': 'https://artic.edu/artworks/%(id)s' % result,
+            'title': result['title'] + " (%(date_display)s) //  %(artist_display)s" % result,
+            'content': result['medium_display'],
+            'author': ', '.join(result['artist_titles']),
+            'img_src': image_api + '/%(image_id)s/full/843,/0/default.jpg' % result,
+            'img_format': result['dimensions'],
+            'template': 'images.html'
+        })
+
+    return results