[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/nosql-engines.rst

@@ -0,0 +1,97 @@
+.. _nosql engines:
+
+===============
+NoSQL databases
+===============
+
+.. sidebar:: further read
+
+   - `NoSQL databases <https://en.wikipedia.org/wiki/NoSQL>`_
+   - `redis.io <https://redis.io/>`_
+   - `MongoDB <https://www.mongodb.com>`_
+
+.. contents::
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+.. sidebar:: info
+
+   Initial sponsored by `Search and Discovery Fund
+   <https://nlnet.nl/discovery>`_ of `NLnet Foundation <https://nlnet.nl/>`_.
+
+The following `NoSQL databases`_ are supported:
+
+- :ref:`engine redis_server`
+- :ref:`engine mongodb`
+
+All of the engines above are just commented out in the :origin:`settings.yml
+<searx/settings.yml>`, as you have to set various options and install
+dependencies before using them.
+
+By default, the engines use the ``key-value`` template for displaying results /
+see :origin:`simple <searx/templates/simple/result_templates/key-value.html>`
+theme.  If you are not satisfied with the original result layout, you can use
+your own template, set ``result_template`` attribute to ``{template_name}`` and
+place the templates at::
+
+  searx/templates/{theme_name}/result_templates/{template_name}
+
+Furthermore, if you do not wish 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`.
+
+
+Extra Dependencies
+==================
+
+For using :ref:`engine redis_server` or :ref:`engine mongodb` you need to
+install additional packages in Python's Virtual Environment of your SearXNG
+instance.  To switch into the environment (:ref:`searxng-src`) you can use
+:ref:`searxng.sh`::
+
+  $ sudo utils/searxng.sh instance cmd bash
+  (searxng-pyenv)$ pip install ...
+
+
+Configure the engines
+=====================
+
+`NoSQL databases`_ are used for storing arbitrary data without first defining
+their structure.
+
+
+.. _engine redis_server:
+
+Redis Server
+------------
+
+.. _redis: https://github.com/andymccurdy/redis-py#installation
+
+.. sidebar:: info
+
+   - ``pip install`` redis_
+   - redis.io_
+   - :origin:`redis_server.py <searx/engines/redis_server.py>`
+
+.. automodule:: searx.engines.redis_server
+  :members:
+
+
+.. _engine mongodb:
+
+MongoDB
+-------
+
+.. _pymongo: https://github.com/mongodb/mongo-python-driver#installation
+
+.. sidebar:: info
+
+   - ``pip install`` pymongo_
+   - MongoDB_
+   - :origin:`mongodb.py <searx/engines/mongodb.py>`
+
+
+.. automodule:: searx.engines.mongodb
+  :members:
+