[feat] engine: add adobe stock video and audio engines

The engine has been revised; there is now the option ``adobe_content_types``
with which it is possible to configure engines for video and audio from the
adobe stock.  BTW this patch adds documentation to the engine.

To test all three engines in one use a search term like::

    !asi !asv !asa sound

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

searx/settings.yml

@@ -327,9 +327,32 @@ engines:
 
   - name: adobe stock
     engine: adobe_stock
-    # available search orders: 'relevant', 'featured', 'creation', 'nb_downloads'
-    # adobe_order: relevance
-    shortcut: as
+    shortcut: asi
+    categories: ["images"]
+    # https://docs.searxng.org/dev/engines/online/adobe_stock.html
+    adobe_order: relevance
+    adobe_content_types: ["photo", "illustration", "zip_vector", "template", "3d", "image"]
+    timeout: 6
+    disabled: true
+
+  - name: adobe stock video
+    engine: adobe_stock
+    shortcut: asv
+    network: adobe stock
+    categories: ["videos"]
+    adobe_order: relevance
+    adobe_content_types: ["video"]
+    timeout: 6
+    disabled: true
+
+  - name: adobe stock audio
+    engine: adobe_stock
+    shortcut: asa
+    network: adobe stock
+    categories: ["music"]
+    adobe_order: relevance
+    adobe_content_types: ["audio"]
+    timeout: 6
     disabled: true
 
   - name: alpine linux packages