[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/command-line-engines.rst

@@ -0,0 +1,23 @@
+.. _engine command:
+
+====================
+Command Line Engines
+====================
+
+.. sidebar:: info
+
+   - :origin:`command.py <searx/engines/command.py>`
+   - :ref:`offline engines`
+
+.. 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/>`_.
+
+.. automodule:: searx.engines.command
+  :members: