doc: add 'Architecture' article to admin section

Herein we add some hints and suggestions about typical architectures of
searx infrastructures.  We start with a contribution from @dalf

- https://github.com/asciimoo/searx/pull/1776#issuecomment-567917320



thanks @dalf !!

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