Add disk format content from Image Guide
Add disk format content from Image Guide

The container_format and disk_format identifiers and their
descriptions are being maintained in the Glance docs.  Move some
useful content on this topic over from the Image Guide before
deleting it over there so it's only being maintained in one
location.

Change-Id: Ifa8de9d3a3a2aa78ee305aaebb23cd982b02dd91
Partial-bug: #1842343

openstack/glance,openstack/glance,openstack/glance