Rearranges to create new Compute API Guide

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

api-guide/source/microversions.rst

@@ -0,0 +1,21 @@
+..
+      Licensed under the Apache License, Version 2.0 (the "License"); you may
+      not use this file except in compliance with the License. You may obtain
+      a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+      Unless required by applicable law or agreed to in writing, software
+      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+      License for the specific language governing permissions and limitations
+      under the License.
+
+==============
+Microversions
+==============
+
+API v2.1 users micro versions, instead of extensions, to allow for the API
+to slowly evolve over time, without breaking any existing users.
+
+TODO - add more information.