Add Watcher API reference

This patch set adds API reference[1]
1. https://review.openstack.org/#/c/558559/

Change-Id: Ic91c09bdccfe533b53e280f08f178b89b610a269

www/project-data/latest.yaml

@@ -398,6 +398,7 @@
   has_admin_guide: true
   has_user_guide: true
   has_config_ref: true
+  has_api_ref: true
 - name: python-watcherclient
   service: Infrastructure Optimization service Python Bindings
   type: service-client