Add oslo.db link to the documentation
Add oslo.db link to the documentation

oslo.db has been graduated from oslo-incubator, so we should add
required link to the oslo developers documentation.

blueprint oslo-db-lib

Change-Id: Ib9d1b13d8d303a737d3aec8cefb0a11b2513dd58

potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,gongysh2004/openstack-manual,openstack/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,Akasurde/openstack-manuals