[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/settings_brand.rst

@@ -0,0 +1,25 @@
+.. _settings brand:
+
+==========
+``brand:``
+==========
+
+.. code:: yaml
+
+   brand:
+     issue_url: https://github.com/searxng/searxng/issues
+     docs_url: https://docs.searxng.org
+     public_instances: https://searx.space
+     wiki_url: https://github.com/searxng/searxng/wiki
+
+``issue_url`` :
+  If you host your own issue tracker change this URL.
+
+``docs_url`` :
+  If you host your own documentation change this URL.
+
+``public_instances`` :
+  If you host your own https://searx.space change this URL.
+
+``wiki_url`` :
+  Link to your wiki (or ``false``)