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>

docs/admin/arch_public.dot

+digraph G {
+
+  node [style=filled, shape=box, fillcolor="#ffffcc", fontname="Sans"];
+  edge [fontname="Sans"];
+
+  browser [label="Browser", shape=Mdiamond];
+  rp      [label="Reverse Proxy", href="url to configure reverse proxy"];
+  filtron [label="Filtron",       href="https://github.com/asciimoo/filtron"];
+  morty   [label="Morty",         href="https://github.com/asciimoo/morty"];
+  static  [label="Static files",  href="url to configure static files"];
+  uwsgi   [label="uwsgi",         href="url to configure uwsgi"]
+  searx1  [label="Searx #1"];
+  searx2  [label="Searx #2"];
+  searx3  [label="Searx #3"];
+  searx4  [label="Searx #4"];
+
+  browser -> rp [label="HTTPS"]
+
+  subgraph cluster_searx {
+      label = "Searx instance" fontname="Sans";
+      bgcolor="#fafafa";
+      { rank=same; static rp };
+      rp -> morty      [label="optional: images and HTML pages proxy"];
+      rp -> static     [label="optional: reverse proxy serves directly static files"];
+      rp -> filtron    [label="HTTP"];
+      filtron -> uwsgi [label="HTTP"];
+      uwsgi -> searx1;
+      uwsgi -> searx2;
+      uwsgi -> searx3;
+      uwsgi -> searx4;
+  }
+
+}

docs/admin/architecture.rst

+.. _architecture:
+
+============
+Architecture
+============
+
+.. sidebar:: Needs work!
+
+   This article needs some work / Searx is a collaborative effort.  If you have
+   any contribution, feel welcome to send us your :pull:`PR <../pulls>`, see
+   :ref:`how to contribute`.
+
+Herein you will find some hints and suggestions about typical architectures of
+searx infrastructures.
+
+We start with a contribution from :pull:`@dalf <1776#issuecomment-567917320>`.
+It shows a *reference* setup for public searx instances.
+
+.. _arch public:
+
+.. kernel-figure:: arch_public.dot
+   :alt: arch_public.dot
+
+   Reference architecture of a public searx setup.

docs/admin/index.rst

@@ -7,6 +7,7 @@ Administrator documentation
 
    installation
    api
+   architecture
    filtron
    morty
    engines

docs/dev/contribution_guide.rst

+.. _how to contribute:
+
 =================
 How to contribute
 =================