[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_ui.rst

@@ -0,0 +1,62 @@
+.. _settings ui:
+
+=======
+``ui:``
+=======
+
+.. _cache busting:
+   https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control#caching_static_assets_with_cache_busting
+
+.. code:: yaml
+
+   ui:
+     static_use_hash: false
+     default_locale: ""
+     query_in_title: false
+     infinite_scroll: false
+     center_alignment: false
+     cache_url: https://web.archive.org/web/
+     default_theme: simple
+     theme_args:
+       simple_style: auto
+
+.. _static_use_hash:
+
+``static_use_hash`` :
+  Enables `cache busting`_ of static files.
+
+``default_locale`` :
+  SearXNG interface language.  If blank, the locale is detected by using the
+  browser language.  If it doesn't work, or you are deploying a language
+  specific instance of searx, a locale can be defined using an ISO language
+  code, like ``fr``, ``en``, ``de``.
+
+``query_in_title`` :
+  When true, the result page's titles contains the query it decreases the
+  privacy, since the browser can records the page titles.
+
+``infinite_scroll``:
+  When true, automatically loads the next page when scrolling to bottom of the current page.
+
+``center_alignment`` : default ``false``
+  When enabled, the results are centered instead of being in the left (or RTL)
+  side of the screen.  This setting only affects the *desktop layout*
+  (:origin:`min-width: @tablet <searx/static/themes/simple/src/less/definitions.less>`)
+
+.. cache_url:
+
+``cache_url`` : ``https://web.archive.org/web/``
+  URL prefix of the internet archive or cache, don't forgett trailing slash (if
+  needed).  The default is https://web.archive.org/web/ alternatives are:
+
+  - https://webcache.googleusercontent.com/search?q=cache:
+  - https://archive.today/
+
+``default_theme`` :
+  Name of the theme you want to use by default on your SearXNG instance.
+
+``theme_args.simple_style``:
+  Style of simple theme: ``auto``, ``light``, ``dark``
+
+``results_on_new_tab``:
+  Open result links in a new tab by default.