doc: Add overview diagram

Signed-off-by: Rahix <rahix@rahix.de>

Documentation/index.rst

@@ -3,9 +3,16 @@ card10 firmware docs
 
 This is the API documentation for card10's default firmware.
 
+Overview
+--------
+
+The design roughly looks like this:
+
+.. image:: static/overview.svg
+
 .. toctree::
    :maxdepth: 2
-   :caption: Contents:
+   :caption: Contents
 
    epicardium
    pycardium