rel6 install documentation missing

the url http://docs.observium.org/install_rhel6/ leads to a 404 page not found.
there seems to be no way to find the page or install instructions for rhel/centos6

the http://docs.observium.org/ link to 'REHL/Centos install' defaults to http://docs.observium.org/install_rhel7/ with no indication how usable this is in reference to RHEL/centos6. 

The nearest usable guide found is https://hostingwikipedia.com/install-observium-centos-6-7/ which is to Install Observium on CentOS 6.7.