rework index intro to describe nova
rework index intro to describe nova

The index previously was written for the developer docs, but this is
more than developer docs now. That also means this is the general
google landing page for people, so a basic "what is nova" is
appropriate.

Part of bp: doc-migration

Change-Id: I5fdafd9f6cf07a19bf86a6343663dad410887dcb

jianghuaw/nova,openstack/nova,phenoxim/nova,phenoxim/nova,gooddata/openstack-nova,vmturbo/nova,gooddata/openstack-nova,klmitch/nova,gooddata/openstack-nova,mikalstill/nova,mikalstill/nova,vmturbo/nova,klmitch/nova,rahulunair/nova,mahak/nova,rahulunair/nova,rahulunair/nova,klmitch/nova,mikalstill/nova,mahak/nova,jianghuaw/nova,vmturbo/nova,klmitch/nova,openstack/nova,vmturbo/nova,mahak/nova,jianghuaw/nova,gooddata/openstack-nova,openstack/nova,jianghuaw/nova