[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_general.rst

@@ -0,0 +1,34 @@
+.. _settings general:
+
+============
+``general:``
+============
+
+.. code:: yaml
+
+   general:
+     debug: false
+     instance_name:  "SearXNG"
+     privacypolicy_url: false
+     donation_url: false
+     contact_url: false
+     enable_metrics: true
+
+``debug`` : ``$SEARXNG_DEBUG``
+  Allow a more detailed log if you run SearXNG directly. Display *detailed* error
+  messages in the browser too, so this must be deactivated in production.
+
+``donation_url`` :
+  Set value to ``true`` to use your own donation page written in the
+  :ref:`searx/info/en/donate.md <searx.infopage>` and use ``false`` to disable
+  the donation link altogether.
+
+``privacypolicy_url``:
+  Link to privacy policy.
+
+``contact_url``:
+  Contact ``mailto:`` address or WEB form.
+
+``enable_metrics``:
+  Enabled by default. Record various anonymous metrics availabled at ``/stats``,
+  ``/stats/errors`` and ``/preferences``.