Fix link from doc index to user guide
Fix link from doc index to user guide

Change-Id: I6687e5c4f3c6798b93bf8e3f897511980658fb7f

openstack/ironic,NaohiroTamura/ironic,SauloAislan/ironic,pshchelo/ironic,NaohiroTamura/ironic,SauloAislan/ironic,pshchelo/ironic,openstack/ironic