[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>

searx/engines/command.py

@@ -1,6 +1,77 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
-"""
- Command (offline)
+"""With *command engines* administrators can run engines to integrate arbitrary
+shell commands.
+
+.. attention::
+
+   When creating and enabling a ``command`` engine on a public instance, you
+   must be careful to avoid leaking private data.
+
+The easiest solution is to limit the access by setting ``tokens`` as described
+in section :ref:`private engines`.  The engine base is flexible.  Only your
+imagination can limit the power of this engine (and maybe security concerns).
+
+Configuration
+=============
+
+The following options are available:
+
+``command``:
+  A comma separated list of the elements of the command.  A special token
+  ``{{QUERY}}`` tells where to put the search terms of the user. Example:
+
+  .. code:: yaml
+
+     ['ls', '-l', '-h', '{{QUERY}}']
+
+``delimiter``:
+  A mapping containing a delimiter ``char`` and the *titles* of each element in
+  ``keys``.
+
+``parse_regex``:
+  A dict containing the regular expressions for each result key.
+
+``query_type``:
+
+  The expected type of user search terms.  Possible values: ``path`` and
+  ``enum``.
+
+  ``path``:
+    Checks if the user provided path is inside the working directory.  If not,
+    the query is not executed.
+
+  ``enum``:
+    Is a list of allowed search terms.  If the user submits something which is
+    not included in the list, the query returns an error.
+
+``query_enum``:
+  A list containing allowed search terms if ``query_type`` is set to ``enum``.
+
+``working_dir``:
+  The directory where the command has to be executed.  Default: ``./``.
+
+``result_separator``:
+  The character that separates results. Default: ``\\n``.
+
+Example
+=======
+
+The example engine below can be used to find files with a specific name in the
+configured working directory:
+
+.. code:: yaml
+
+  - name: find
+    engine: command
+    command: ['find', '.', '-name', '{{QUERY}}']
+    query_type: path
+    shortcut: fnd
+    delimiter:
+        chars: ' '
+        keys: ['line']
+
+Implementations
+===============
 """
 
 import re