Clean up keystone doc landing page
Clean up keystone doc landing page

- update the API link
- add a link to keystoneauth
- include a link to the docs source

Change-Id: Id1e5e00164c4ba152bbd9fff9ac39f309be8e5c3
Closes-Bug: 1648886

ilay09/keystone,rajalokan/keystone,openstack/keystone,rajalokan/keystone,openstack/keystone,openstack/keystone,ilay09/keystone,rajalokan/keystone,mahak/keystone,ilay09/keystone,mahak/keystone,mahak/keystone