Backbone IP addresses automatic update

Changes to the Backbone documentation to make the update of IP addresses easier.

• Added the macros plugin to use global variables - @jandt-desy let me know if you know of sthg less "invasive"
• From now on, only the backbone-ip-addresses.yaml file will need to be pushed when there is a modification to the IP addresses announced in the Backbone
• Maybe worth triggering an automatic MR when this file is pushed - like these guys here (but since then there may be a more elegant way) - will ask around.