[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_server.rst

@@ -0,0 +1,54 @@
+.. _settings server:
+
+===========
+``server:``
+===========
+
+.. code:: yaml
+
+   server:
+       base_url: http://example.org/location  # change this!
+       port: 8888
+       bind_address: "127.0.0.1"
+       secret_key: "ultrasecretkey"           # change this!
+       limiter: false
+       image_proxy: false
+       default_http_headers:
+         X-Content-Type-Options : nosniff
+         X-XSS-Protection : 1; mode=block
+         X-Download-Options : noopen
+         X-Robots-Tag : noindex, nofollow
+         Referrer-Policy : no-referrer
+
+
+``base_url`` : ``$SEARXNG_URL`` :ref:`buildenv <make buildenv>`
+  The base URL where SearXNG is deployed.  Used to create correct inbound links.
+  If you change the value, don't forget to rebuild instance's environment
+  (:ref:`utils/brand.env <make buildenv>`)
+
+``port`` & ``bind_address``: ``$SEARXNG_PORT`` & ``$SEARXNG_BIND_ADDRESS`` :ref:`buildenv <make buildenv>`
+  Port number and *bind address* of the SearXNG web application if you run it
+  directly using ``python searx/webapp.py``.  Doesn't apply to a SearXNG
+  services running behind a proxy and using socket communications.  If you
+  change the value, don't forget to rebuild instance's environment
+  (:ref:`utils/brand.env <make buildenv>`)
+
+``secret_key`` : ``$SEARXNG_SECRET``
+  Used for cryptography purpose.
+
+.. _limiter:
+
+``limiter`` :
+  Rate limit the number of request on the instance, block some bots.  The
+  :ref:`limiter src` requires a :ref:`settings redis` database.
+
+.. _image_proxy:
+
+``image_proxy`` :
+  Allow your instance of SearXNG of being able to proxy images.  Uses memory space.
+
+.. _HTTP headers: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers
+
+``default_http_headers`` :
+  Set additional HTTP headers, see `#755 <https://github.com/searx/searx/issues/715>`__
+