Add OpenAPI related projects under the OpenStackSDK

- add `codegenerator` repository used to generate OpenAPI specs from
  service sources and client API bindings from the OpenAPI
- add `openapi` repository with documents describing OpenStack
  flavouring of OpenAPI and Sphinx extension to render them similar to
  os-api-ref
- slightly adapt the overall team mission to better match the reality

Depends-On: https://review.opendev.org/c/openstack/project-config/+/910580
Change-Id: I3513c3363eed62155291f2d9bf094bf07725df64

reference/projects.yaml

@@ -1838,14 +1838,21 @@ OpenStackSDK:
   irc-channel: openstack-sdks
   service: Multi-cloud Python SDK and CLI for End Users
   mission: >
-    To provide a unified multi-cloud aware Python SDK and CLI for the OpenStack
+    To provide a unified multi-cloud aware SDK and CLI for the OpenStack
     REST API exposing both the full set of low-level APIs as well as curated higher
-    level business logic.
+    level business logic. Ensure end user is having necessary tools to work
+    with OpenStack based clouds.
   url: https://docs.openstack.org/openstacksdk/latest/
   deliverables:
     cliff:
       repos:
         - openstack/cliff
+    codegenerator:
+      repos:
+        - openstack/codegenerator
+    openapi:
+      repos:
+        - openstack/openapi
     openstackclient:
       repos:
         - openstack/openstackclient