[mod] themes/simple/img/searxng.svg -> src/brand/searxng.svg

* move `searx/static/themes/simple/img/searxng.svg` to `src/brand/searxng.svg`

* README.rst can use it without a reference to a theme.

* the simple theme can create `searx/static/themes/simple/img/searxng.png` using
  the svg2png task

Suggested-by: @dalf https://github.com/searxng/searxng/pull/561#issuecomment-981747902


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