[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_redis.rst

@@ -0,0 +1,43 @@
+.. _settings redis:
+
+==========
+``redis:``
+==========
+
+.. _Redis.from_url(url): https://redis-py.readthedocs.io/en/stable/connections.html#redis.client.Redis.from_url
+
+A redis DB can be connected by an URL, in :py:obj:`searx.redisdb` you
+will find a description to test your redis connection in SerXNG.  When using
+sockets, don't forget to check the access rights on the socket::
+
+  ls -la /usr/local/searxng-redis/run/redis.sock
+  srwxrwx--- 1 searxng-redis searxng-redis ... /usr/local/searxng-redis/run/redis.sock
+
+In this example read/write access is given to the *searxng-redis* group.  To get
+access rights to redis instance (the socket), your SearXNG (or even your
+developer) account needs to be added to the *searxng-redis* group.
+
+``url`` : ``$SEARXNG_REDIS_URL``
+  URL to connect redis database, see `Redis.from_url(url)`_ & :ref:`redis db`::
+
+    redis://[[username]:[password]]@localhost:6379/0
+    rediss://[[username]:[password]]@localhost:6379/0
+    unix://[[username]:[password]]@/path/to/socket.sock?db=0
+
+.. admonition:: Tip for developers
+
+   To set up a local redis instance, first set the socket path of the Redis DB
+   in your YAML setting:
+
+   .. code:: yaml
+
+      redis:
+        url: unix:///usr/local/searxng-redis/run/redis.sock?db=0
+
+   Then use the following commands to install the redis instance ::
+
+     $ ./manage redis.build
+     $ sudo -H ./manage redis.install
+     $ sudo -H ./manage redis.addgrp "${USER}"
+     # don't forget to logout & login to get member of group
+