[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>

docs/dev/engines/online/tavily.rst

@@ -0,0 +1,8 @@
+.. _tavily engine:
+
+======
+Tavily
+======
+
+.. automodule:: searx.engines.tavily
+   :members: