[www] Add German API guide to German index
[www] Add German API guide to German index

The guide is translated and published, link to it.

Change-Id: I556efb9f183c920a9399f2a4c1643f0fcf5f312c

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals