[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/utils/index.rst

@@ -10,7 +10,6 @@ and developers.
 
 .. toctree::
    :maxdepth: 2
-   :caption: Contents
 
    searxng.sh
    lxc.sh

docs/utils/lxc.sh.rst

@@ -26,7 +26,7 @@ to care about*).
    - `LXC/LXD Image Server`_
    - `LXD@github`_
 
-.. contents:: Contents
+.. contents::
    :depth: 2
    :local:
    :backlinks: entry

docs/utils/searxng.sh.rst

@@ -15,7 +15,7 @@ script :origin:`utils/searxng.sh`.
    - :ref:`installation nginx`
    - :ref:`installation apache`
 
-.. contents:: Contents
+.. contents::
    :depth: 2
    :local:
    :backlinks: entry