[website sprint] include the structural diagram on the main page (#1494)

* include the structural diagram on the main page

* fix alt on arch diagram

sphinx_doc/index.rst

@@ -32,6 +32,11 @@ Here you will find documentation on how to install and use |PN| with a Turtlebot
 3, as well as how to customize it for other robots, tune the behavior for better
 performance, as well as customize the internals for advanced results.
 
+.. image:: images/architectural_diagram.png
+    :height: 720px
+    :width: 1024px
+    :alt: Navigation2 Block Diagram
+
 Sections
 ********