[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/admin/index.rst

@@ -4,8 +4,8 @@ Administrator documentation
 
 .. toctree::
    :maxdepth: 2
-   :caption: Contents
 
+   settings/index
    installation
    installation-docker
    installation-scripts
@@ -15,7 +15,6 @@ Administrator documentation
    installation-apache
    update-searxng
    answer-captcha
-   engines/index
    api
    architecture
    plugins