[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/user/configured_engines.rst

@@ -22,7 +22,7 @@ Configured Engines
    called *tabs*), engines can be queried by their name or the categories they
    belong to, by using a :ref:`\!bing syntax <search-syntax>`.
 
-.. contents:: Contents
+.. contents::
    :depth: 2
    :local:
    :backlinks: entry

docs/user/index.rst

@@ -2,7 +2,7 @@
 User information
 ================
 
-.. contents:: Contents
+.. contents::
    :depth: 3
    :local:
    :backlinks: entry