[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.rst

@@ -0,0 +1,117 @@
+.. _settings.yml:
+
+================
+``settings.yml``
+================
+
+This page describe the options possibilities of the :origin:`searx/settings.yml`
+file.
+
+.. sidebar:: Further reading ..
+
+   - :ref:`use_default_settings.yml`
+   - :ref:`search API`
+
+.. contents::
+   :depth: 2
+   :local:
+   :backlinks: entry
+
+.. _settings location:
+
+settings.yml location
+=====================
+
+The initial ``settings.yml`` we be load from these locations:
+
+1. the full path specified in the ``SEARXNG_SETTINGS_PATH`` environment variable.
+2. ``/etc/searxng/settings.yml``
+
+If these files don't exist (or are empty or can't be read), SearXNG uses the
+:origin:`searx/settings.yml` file.  Read :ref:`settings use_default_settings` to
+see how you can simplify your *user defined* ``settings.yml``.
+
+
+
+.. _settings use_default_settings:
+
+use_default_settings
+====================
+
+.. sidebar:: ``use_default_settings: true``
+
+   - :ref:`settings location`
+   - :ref:`use_default_settings.yml`
+   - :origin:`/etc/searxng/settings.yml <utils/templates/etc/searxng/settings.yml>`
+
+The user defined ``settings.yml`` is loaded from the :ref:`settings location`
+and can relied on the default configuration :origin:`searx/settings.yml` using:
+
+ ``use_default_settings: true``
+
+``server:``
+  In the following example, the actual settings are the default settings defined
+  in :origin:`searx/settings.yml` with the exception of the ``secret_key`` and
+  the ``bind_address``:
+
+  .. code:: yaml
+
+    use_default_settings: true
+    server:
+        secret_key: "ultrasecretkey"   # change this!
+        bind_address: "0.0.0.0"
+
+``engines:``
+  With ``use_default_settings: true``, each settings can be override in a
+  similar way, the ``engines`` section is merged according to the engine
+  ``name``.  In this example, SearXNG will load all the default engines, will
+  enable the ``bing`` engine and define a :ref:`token <private engines>` for
+  the arch linux engine:
+
+  .. code:: yaml
+
+    use_default_settings: true
+    server:
+      secret_key: "ultrasecretkey"   # change this!
+    engines:
+      - name: arch linux wiki
+        tokens: ['$ecretValue']
+      - name: bing
+        disabled: false
+
+
+``engines:`` / ``remove:``
+  It is possible to remove some engines from the default settings. The following
+  example is similar to the above one, but SearXNG doesn't load the the google
+  engine:
+
+  .. code:: yaml
+
+    use_default_settings:
+      engines:
+        remove:
+          - google
+    server:
+      secret_key: "ultrasecretkey"   # change this!
+    engines:
+      - name: arch linux wiki
+        tokens: ['$ecretValue']
+
+``engines:`` / ``keep_only:``
+  As an alternative, it is possible to specify the engines to keep. In the
+  following example, SearXNG has only two engines:
+
+  .. code:: yaml
+
+    use_default_settings:
+      engines:
+        keep_only:
+          - google
+          - duckduckgo
+    server:
+      secret_key: "ultrasecretkey"   # change this!
+    engines:
+      - name: google
+        tokens: ['$ecretValue']
+      - name: duckduckgo
+        tokens: ['$ecretValue']