Add Identity API references to main page
Add Identity API references to main page

The "API References" section on http://docs.openstack.org/ was
missing some Identity docs such as a link to the v2.0 Admin API
reference.

Change-Id: I1885aab838a07889e4b83643a01e629f61d78551
Closes-Bug: 1480670

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,potsmaster/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,saeki-masaki/openstack-manuals,Akasurde/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals