Rework the old logo

* Re-work the old banner logo for the Sphinx docs
* Remove the remaining leftover assets from the old web site

So this is one possibility, I retained the other logo files so we have
options to play with.

Change-Id: Iac15899780072450dc30600a719283c0e2b0c0ae
diff --git a/doc/source/index.rst b/doc/source/index.rst
index dbefdec..69e6ce9 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -1,6 +1,8 @@
 DevStack - an OpenStack Community Production
 ============================================
 
+.. image:: assets/images/logo-blue.png
+
 .. toctree::
    :glob:
    :maxdepth: 1