[feat] allow customization of logo and favicon

also removed searxng-wordmark from templates directory and build
files, because it is no longer used anywhere

docs/admin/settings/settings_brand.rst

@@ -11,6 +11,10 @@
      docs_url: https://docs.searxng.org
      public_instances: https://searx.space
      wiki_url: https://github.com/searxng/searxng/wiki
+     custom_files:
+       logo: /path/to/file.png
+       favicon_png: /path/to/file.png
+       favicon_svg: /path/to/file.svg
 
 ``issue_url`` :
   If you host your own issue tracker change this URL.
@@ -23,3 +27,12 @@
 
 ``wiki_url`` :
   Link to your wiki (or ``false``)
+
+``custom_files.logo`` :
+  Filepath to a custom logo. Be sure it has the right file extension, as that's used to determine the mimetype.
+
+``custom_files.favicon_png`` :
+  Filepath to a custom PNG favicon.
+
+``custom_files.favicon_svg`` :
+  Filepath to a custom SVG favicon. When using, be sure to also set ``custom_files.favicon_png``, as some browsers still don't support SVG favicons.