Provide detailed information on architecture

Added new global architecture diagram using Dia tool
and a more detailed description of each component.
Exported this diagram in SVG format.
The source code of the diagram is stored in
/doc/images_src/ folder.
Moved also the architecture.rst file from the /dev
folder to the root folder of the Watcher doc.

Change-Id: I74379390178673dcb6a043967b31e38ca9560bb3

doc/source/index.rst

@@ -22,7 +22,7 @@ Introduction
   :maxdepth: 1
 
   dev/glossary
-  dev/architecture
+  architecture
   dev/environment
   dev/contributing
   dev/plugins