[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/settings/index.rst

@@ -0,0 +1,25 @@
+========
+Settings
+========
+
+.. sidebar:: Further reading ..
+
+   - :ref:`engine settings`
+   - :ref:`engine file`
+
+.. toctree::
+   :maxdepth: 2
+
+   settings
+   settings_engine
+   settings_brand
+   settings_general
+   settings_search
+   settings_server
+   settings_ui
+   settings_redis
+   settings_outgoing
+   settings_categories_as_tabs
+
+
+