[mod] brand - partial migration of settings to msgspec.Struct (#5280)

The settings are currently an untyped key/value structure, whose types are
dynamically built at runtime.  The construction process of this structure
is *hand-crafted*.

In the long term, we want a static typing of this structure, based on a standard
tool.  The ``msgspec.Struct`` structures are suitable as a standard tool.

This patch makes a first step towards static typing and implements the "brand"
section using ``msgspec.Struct`` structures.

BTW: searx/settings_defaults.py - ``git_url`` and ``git_branch`` had been
removed in aee613d256, this is a leftover.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/admin/settings/settings_brand.rst

@@ -4,22 +4,5 @@
 ``brand:``
 ==========
 
-.. code:: yaml
-
-   brand:
-     issue_url: https://github.com/searxng/searxng/issues
-     docs_url: https://docs.searxng.org
-     public_instances: https://searx.space
-     wiki_url: https://github.com/searxng/searxng/wiki
-
-``issue_url`` :
-  If you host your own issue tracker change this URL.
-
-``docs_url`` :
-  If you host your own documentation change this URL.
-
-``public_instances`` :
-  If you host your own https://searx.space change this URL.
-
-``wiki_url`` :
-  Link to your wiki (or ``false``)
+.. autoclass:: searx.brand.SettingsBrand
+   :members: