[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>

searx/engines/postgresql.py

@@ -1,12 +1,33 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # lint: pylint
-"""PostgreSQL database (offline)
+"""PostgreSQL is a powerful and robust open source database.  Before configuring
+the PostgreSQL engine, you must install the dependency ``psychopg2``.
+
+Example
+=======
+
+Below is an example configuration:
+
+.. code:: yaml
+
+   - name: my_database
+     engine: postgresql
+     database: my_database
+     username: searxng
+     password: password
+     query_str: 'SELECT * from my_table WHERE my_column = %(query)s'
+
+Implementations
+===============
 
 """
 
-# import error is ignored because the admin has to install mysql manually to use
-# the engine
-import psycopg2  # pyright: ignore # pylint: disable=import-error
+try:
+    import psycopg2  # type: ignore
+except ImportError:
+    # import error is ignored because the admin has to install postgresql
+    # manually to use the engine.
+    pass
 
 engine_type = 'offline'
 host = "127.0.0.1"