XML API interface has been already removed from Nova completely,
and its description of the extension concept becomes meaningless now.
So this patch removes it.
A small amount of the JSON description is left. The details are
trimmed down, as we're actually trying to get people away from
extensions as a concept.
A big warning to that effect is added into the document.
Partially implements blueprint complete-todo-in-api-concept-doc
Change-Id: Icf8bf87f38733ba03cf5ce1225594b0b282fde56
Uses openstackdocstheme to match other content
Has a dependent change in project-config also so that
file will build to developer.openstack.org/compute
at https://review.openstack.org/#/c/231000/
Change-Id: Ic060a1e79e4b2f8695cb788ff4df018e0cfd3286
Ken'ichi Ohmichi