[brand] initial - folder src/brand for searxng-wordmark

Folder src/brand is intended to place logo, wordmark and more in.  This commit
moves the origin (source) of searxng-wordmark.svg from the simple theme into
this folder

About the new created src/ folder:

On the long term we can move all the files from static/themes/<name>/src/* into
a src/themes/<name>/* folder.

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