[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/offline/search-indexer-engines.rst

@@ -0,0 +1,62 @@
+=================
+Local Search APIs
+=================
+
+.. sidebar:: further read
+
+   - `Comparison to alternatives
+     <https://docs.meilisearch.com/learn/what_is_meilisearch/comparison_to_alternatives.html>`_
+
+.. contents::
+   :depth: 1
+   :local:
+   :backlinks: entry
+
+.. sidebar:: info
+
+   Initial sponsored by `Search and Discovery Fund
+   <https://nlnet.nl/discovery>`_ of `NLnet Foundation <https://nlnet.nl/>`_.
+
+Administrators might find themselves wanting to integrate locally running search
+engines.  The following ones are supported for now:
+
+* `Elasticsearch`_
+* `Meilisearch`_
+* `Solr`_
+
+Each search engine is powerful, capable of full-text search.  All of the engines
+above are added to ``settings.yml`` just commented out, as you have to
+``base_url`` for all them.
+
+Please note that if you are not using HTTPS to access these engines, you have to
+enable HTTP requests by setting ``enable_http`` to ``True``.
+
+Furthermore, if you do not want to expose these engines on a public instance,
+you can still add them and limit the access by setting ``tokens`` as described
+in section :ref:`private engines`.
+
+.. _engine meilisearch:
+
+MeiliSearch
+===========
+
+.. automodule:: searx.engines.meilisearch
+  :members:
+
+
+.. _engine elasticsearch:
+
+Elasticsearch
+=============
+
+.. automodule:: searx.engines.elasticsearch
+  :members:
+
+.. _engine solr:
+
+Solr
+====
+
+.. automodule:: searx.engines.solr
+  :members:
+