add check links and remove the old training link

docs/index.md

@@ -9,8 +9,6 @@ Finally, users will get a better understanding about our choices around the supp
 
 A number of links to more specific CERN trainings will be given when necessary.
 
-The latest version of the training material will always be found at <https://cern.ch/linuxtraining/>
-
 ## What this guide will not do
 
 * This guide will not mention unsupported distributions. It will focus on the newest supported version at CERN: i.e. _CERN CentOS 7_.