[mod] add doc to tavily and slightly improve the engine

- Config options like ``search_type`` renamed to follow the upstream
  API (``topic``).
- Default ``max_results`` is set to 5
- use image description if one exists
- add an init function to check engine's settings
- settings example: additional category 'ai'

To review the added documentation of this path::

    make docs.live

and jump to: http://0.0.0.0:8000/dev/engines/online/tavily.html

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

searx/settings.yml

@@ -1828,21 +1828,28 @@ engines:
     shortcut: tm
     disabled: true
 
+  # Tavily requires an API key as well as other configurations. Before you
+  # activate these engines you should read the documentation.
+  # --> https://docs.searxng.org/dev/engines/online/tavily.html
+  #
   # - name: tavily
   #   engine: tavily
   #   shortcut: tav
-  #   categories: general
-  #   # API key required, see: https://docs.tavily.com/docs/welcome#getting-started
-  #   api_key: 'unset'
-  #   include_images: false
-  #   timeout: 15.0
+  #   categories: [general, ai]
+  #   api_key: unset
+  #   topic: general
+  #   include_images: true
+  #   timeout: 15
+  #   disabled: true
+  #
   # - name: tavily news
   #   engine: tavily
   #   shortcut: tavnews
-  #   categories: news
-  #   api_key: 'unset'
-  #   search_type: news
-  #   timeout: 15.0
+  #   categories: [news, ai]
+  #   api_key: unset
+  #   topic: news
+  #   timeout: 15
+  #   disabled: true
 
   # Requires Tor
   - name: torch