[doc] rearranges Settings & Engines docs for better readability

We have built up detailed documentation of the *settings* and the *engines* over
the past few years.  However, this documentation was still spread over various
chapters and was difficult to navigate in its entirety.

This patch rearranges the Settings & Engines documentation for better
readability.

To review new ordered docs::

   make docs.clean docs.live

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

docs/dev/engines/index.rst

@@ -0,0 +1,97 @@
+.. _engine implementations:
+
+======================
+Engine Implementations
+======================
+
+Framework Components
+====================
+
+.. toctree::
+   :maxdepth: 2
+
+   enginelib
+   engines
+   engine_overview
+
+
+Engine Types
+============
+
+The :py:obj:`engine_type <searx.enginelib.Engine.engine_type>` of an engine
+determines which :ref:`search processor <searx.search.processors>` is used by
+the engine.
+
+In this section a list of the enignes that are documented is given, a complete
+list of the engines can be found in the source under: :origin:`searx/engines`.
+
+.. _online engines:
+
+Online Engines
+--------------
+
+.. sidebar:: info
+
+   - :py:obj:`processors.online <searx.search.processors.online>`
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   demo/demo_online
+   xpath
+   online/*
+
+.. _offline engines:
+
+Offline Engines
+---------------
+
+.. sidebar:: info
+
+   - :py:obj:`processors.offline <searx.search.processors.offline>`
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   offline_concept
+   demo/demo_offline
+   offline/*
+
+.. _online url search:
+
+Online URL Search
+-----------------
+
+.. sidebar:: info
+
+   - :py:obj:`processors.online_url_search <searx.search.processors.online_url_search>`
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   online_url_search/*
+
+.. _online currency:
+
+Online Currency
+---------------
+
+.. sidebar:: info
+
+   - :py:obj:`processors.online_currency <searx.search.processors.online_currency>`
+
+*no engine of this type is documented yet / comming soon*
+
+.. _online dictionary:
+
+Online Dictionary
+-----------------
+
+.. sidebar:: info
+
+   - :py:obj:`processors.online_dictionary <searx.search.processors.online_dictionary>`
+
+*no engine of this type is documented yet / comming soon*