[enh] settings.yml: add use_default_settings option

This change is backward compatible with the existing configurations.

If a settings.yml loaded from an user defined location (SEARX_SETTINGS_PATH or /etc/searx/settings.yml),
then this settings can relied on the default settings.yml with this option:
user_default_settings:True

docs/admin/settings.rst

@@ -206,3 +206,97 @@ Engine settings
 
    A few more options are possible, but they are pretty specific to some
    engines, and so won't be described here.
+
+
+.. _settings location:
+
+settings.yml location
+=====================
+
+First, searx will try to load settings.yml from these locations:
+
+1. the full path specified in the ``SEARX_SETTINGS_PATH`` environment variable.
+2. ``/etc/searx/settings.yml``
+
+If these files don't exist (or are empty or can't be read), searx uses the :origin:`searx/settings.yml` file.
+
+.. _ settings use_default_settings:
+
+use_default_settings
+====================
+
+.. note::
+
+   If searx is cloned from a git repository, most probably there is no need to have an user settings.
+
+The user defined settings.yml can relied on the default configuration :origin:`searx/settings.yml` using ``use_default_settings: True``.
+
+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-block:: yaml
+
+  use_default_settings: true
+  server:
+      secret_key: "uvys6bRhKHUdFF5CqbJonSDSRN8H0sCBziNSrDGNVdpz7IeZhveVart3yvghoKHA"
+  server:
+      bind_address: "0.0.0.0"
+
+With ``use_default_settings: True``, each settings can be override in a similar way with one exception, the ``engines`` section:
+
+* If the ``engines`` section is not defined in the user settings, searx uses the engines from the default setttings (the above example).
+* If the ``engines`` section is defined then:
+
+   * searx loads only the engines declare in the user setttings.
+   * searx merges the configuration according to the engine name.
+
+In the following example, only three engines are available. Each engine configuration is merged with the default configuration.
+
+.. code-block:: yaml
+
+  use_default_settings: true
+  server:
+      secret_key: "uvys6bRhKHUdFF5CqbJonSDSRN8H0sCBziNSrDGNVdpz7IeZhveVart3yvghoKHA"
+  engines:
+    - name: wikipedia
+    - name: wikidata
+    - name: ddg definitions
+
+Another example where four engines are available. The arch linux wiki engine has a :ref:`token<private engines>`.
+
+.. code-block:: yaml
+
+  use_default_settings: true
+  server:
+      secret_key: "uvys6bRhKHUdFF5CqbJonSDSRN8H0sCBziNSrDGNVdpz7IeZhveVart3yvghoKHA"
+  engines:
+    - name: arch linux wiki
+      tokens: ['$ecretValue']
+    - name: wikipedia
+    - name: wikidata
+    - name: ddg definitions
+
+automatic update
+----------------
+
+The following comand creates or updates a minimal user settings (a secret key is defined if it is not already the case):
+
+.. code-block:: sh
+
+  make SEARX_SETTINGS_PATH=/etc/searx/settings.yml user-settings.update
+
+Set ``SEARX_SETTINGS_PATH`` to your user settings path.
+
+As soon the user settings contains an ``engines`` section, it becomes difficult to keep the engine list updated.
+The following command creates or updates the user settings including the ``engines`` section:
+
+.. code-block:: sh
+
+  make SEARX_SETTINGS_PATH=/etc/searx/settings.yml user-settings.update.engines
+
+After that ``/etc/searx/settings.yml``
+
+* has a ``secret key``
+* has a ``engine`` section if it is not already the case, moreover the command:
+
+  * has deleted engines that do not exist in the default settings.
+  * has added engines that exist in the default settings but are not declare in the user settings.

docs/blog/private-engines.rst

@@ -7,6 +7,8 @@ enabled engines on their instances. It might be because they do not want to
 expose some private information through an offline engine. Or they
 would rather share engines only with their trusted friends or colleagues.
 
+.. _private engines:
+
 Private engines
 ===============